Chapter 7

Heredity

Key knowledge

• Transmission of heritable characteristics– Genes as units of inheritance– Eukaryote chromosomes, alleles, prokaryote chromosomes,

plasmids• Cell reproduction: cell cycle, DNA replication, apoptosis,

binary fission, gamete production, inputs and outputs of meiosis

• Variation: genotype, phenotype, continuous variation, discontinuous variation

• Patterns of inheritance– One gene locus – monohybrid cross, including dominance,

recessiveness, codominance, multiple alleles.– Two gene loci – dihybrid cross– Pedigree analysis – autosomal, sex-linked inheritance, test cross

Heredity

• Heredity: is the study of inheritance. Principles of heredity and patterns of inheritance were first established by an Austraian monk, Gregor Mendel.

• Genetics: study of the mechanism and patterns of inheritance through the transmission of coded chemical instructions from one generation to the next.

• Genes: segments of DNA that directed the formation of particular structural and functional protein of cells.

• Genes may code for more than one kind of protein• Genome – the sum aof all the DNA in the cell of an

organism

Reproduction – transmission of heritable traits

Reproduction – transmission of heritable traits

• Cells pass on instructions for growth and development from one generation to the next during mitosis and cytokinesis.

• Living things that originate from one parent are said to reproduce asexually. They usually resemble the parent because they only have one source of hereditary information.

• Organisms that reproduce sexually have two sources of hereditary material, which are carried in specialised reproductive cells called gametes.

Chromosomes and Heredity

• Genetic information carried in its DNA molecules. Eukaryotes in the nucleus

• Prokaryotes – DNA lies free within the cell.

Chromosomes and heredity – Chromosomes of eukaryotes

• Chromatin – DNA & protein

• Nuclear division – DNA molecules appear as double structures each coiled around histone proteins linked at the centromere.

• Chromosomes are normally visible only during cell division. Eukaryotes chromosomes exist in pairs.

Changing shapes of chromosomes

• Metacentric– Centromere in the centre

• Submetacentric– Centromere nearer one end than the other

• Acrocentric– Centromere close to the end

• Telocentric– Centromere on the end

Chromosomes and heredity – Karyotype

• Karyotype – standard form used to display and analyse chromosomes.

• Humans – somatic or body cell – 46 chromosomes which form 23 pairs of which 22 are matched or homologous.

• One chromosome of each pair comes from the male parent via the sperm cell and the other from the female parent via the egg cell (ovum)

• Matched pairs autosomes• 23rd pair is matched in females (XX) but unmatched in

males (XY) is called a heterosome (hetero-different)• Sex chromosomes

Chromosomes and heredity – Karyotype

• Diploid – number of chromosomes in each body (somatic) cell – 2n

• Trait – characteristic

• Locus – the position a gene occupies in a chromosome.

• Allele – alternative form of a gene

Barr Bodies

• In female mammals one of two X chromosomes is inactivated, or turned off and condenses.

Cell division

Cell cycle

• Mitosis – process of nuclear division in somatic cells and cytokinesis, the division of the cell, results in the formation of two diploid daughter cells which contain identical sets of chromosomes.

• Meiosis –nuclear division results in the formation of four daughter cells, which each contain half the number of chromosomes of the original nucleus – haploid.

Cell Cycle

DNA replication

• DNA replication (S Phase) occurs between 2 phases of growth (G1 and G2).

• G1 phase – cells make biochemicals and organelles

• S phases begins with enzyme DNA helicase unzipping the helix of double stranded DNA exposing nucleotide bases. – happens along a small section at a time

• Hydrogen bonds hold two strands of DNA together are weak and the enzyme is easily able to separate them.

DNA Replication

• Junction between the unwound single strands of DNA and the intact double helix is called the replication fork.

• Fork moves along the parental DNA strand so that there is a continuous unwinding of the parental strand.

• Free nucleotides attach to the exposed bases according to the base-pairing rule with the help of DNA polymerase.

• DNA ligase seals the new short stretches of nucleotides into a continuous strand that rewinds.

DNA ReplicationSemi-conservative replication

• Outcome of DNA replication is two double-helix DNA molecules, each consisting of one parental strand and one new strand.

• One of the two strand is conserved from one generation to the next, while the other strand is new.

Nuclear division in Somatic Cells - Mitosis

• Interphase– Chromosomes not visible and cannot be

clearly distinguished.– Immediately before mitosis centrioles visible,

chromatin threads become shorter and thicker – visible under light microscope.

Mitosis• Four main stages of mitosis• Prophase

– During prophase, chromatin threads condenses and sister chromatids become visible – held togheter by a centromere. A spindle forms and the nucleolus disappears from view. The nuclear membrane break downs.

• Metaphase– Chromosomes move to the centre of the cell and line up along the

equator.• Anaphase

– Chromatids separate and move to opposite poles of the spindle. They are chromosomes.

• Telophase– Chromosomes lengthen and become less visible. A new nuclear

envelope forms and nucleoli reform.

Mitosis

• Cytokinesis– Following mitosis, cytokinesis occurs. – Cytoplasm of plant cells divided with formation

of a cell plate which eventually becomes the cell wall.

– Animal cells do not have a cell wall. Cytoplasm divides by a a process called cleavage. Cleavage furrow.

Apoptosis

• Programmed cell death

• Webbing in between fingers.

• Crucial part of the development process

• Apoptosis plays a large role in cell cycle.

• Keeps tight rein on cell division.

Division of Prokaryotic Cells

• Prokaryotic bacterial cells simply replicate their single DNA strand.

• Following replication and separation, a wall forms across the cell and divides into two cells.

• Binary fission.

Nuclear division in sex cells

Meiosis

Meiosis

Stages of Meiosis

Stages of Meiosis – Meiosis I• Prophase I

– Chromosomes condense, nucleolus disappeares, spindle forms. Homologous chromosomes side by side – synapsis. Homologous chromosomes may coil around each other. Later they may move apart by the chromatids remain in contact at points called chiasmata.

• Metaphase I– Nuclear envelope breaks down and the homologous chromosomes move

together to the equator of the spindle.• Anaphase I

– Homologous chromosomes move towards the opposite poles of the spindle. Disjunction of pairs of homologous chromosomes is independent of other chromosomes.

• Telophase I– Cell starts to divide across its middle and nuclear envelopes form around the

two nuclei. The spindle breaks down.• Interphase

– Brief interphase usually occurs. DNA does not duplicate during this interphase

Stages of Meiosis – Meiosis II

• Prophase II– New spindle forms at right angles to the first

• Metaphase II– Chromosomes move to the equator of the spindle

• Anaphase II– Chromatids separate and move apart from each other.

Chromatids become the chromosomes of daughter cells. When they reach the poles, the cells enter

• Telophase II– Spindle disappears, chromosomes regain their thread-like form

and new nuclear envelopes and nucleoli form.

• 4 haploid cells (n)

Mitosis MeiosisNuclear and cell division for growth, repair and replacement of tissues.

Nuclear and cell division for producing sex cell (gametes)

Mitosis takes place in the nucleus of body (somatic) cells.

Meiosis takes place in the gonads or reproductive organs of living things (eg ovaries and testes of mammals, ovaries and anthers of flowering plants, spores of some plants).

One cell division completes the process of mitosis Two cell divisions complete the process of meiosis

Two cells are outputs of the process Four cells (gametes) are outputs of the process

Each daughter cell contains the diploid number of chromosomes (2n)

Each daughter cell contains the haploid number of chromosomes (n)

Asexually reproducing organisms (plant cuttings, runners, bulbs, prokaryotes) reproduce by mitotic division of cells.

Sexually reproducing organisms reproduce by fusion of gametes thus restoring the diploid number (2n) of chromosomes for each cell.

New cells or offspring produced by this kind of reproduction do not show variation between them unless there are environmental influences or mutations; they are genetically identical (ie clones) to each other.

Offspring produced by this kind of reproduction show variation between them.

Variation and diversity of offspring are narrowed Variation and diversity of offspring are increased.

Increasing variation

• Mitosis – offspring have same DNA as parent cell.• Spontaneous or induced changes in DNA mutations

can result in variation of characteristics.• Environmental conditions stable – little variation for

species survival, asexual reproduction is ideal – less energy needed for complex and specialised reproductive processes.

• Sexual reproduction can result in an increase in variation between generations as there is input from two parents.

• If environmental conditions change, some members of a species may have slight variations in characteristics, which may or may not give them a competitive edge in survival.

Patterns of Inheritance

Monohybrid inheritance

• Gene – a segment of DNA that controls a particular trait• Allele – An alternative form of a gene. For example in

pea plants there is a gene which controls pod colours. It has two alleles A – green pods, a – yellow pods

• Dominant – refers to a trait which is expressed when the organism has either one or two copies of the allele present for that particular trait.

• Phenotype – the characteristics shown by a particular organism. The phenotype is a result of an organism’s genotype and any influences due to the environment.

• The genotypes, AA and Aa, would be expected to result in pea pods showing a green phenotype. The genotype aa, would result in pea pods with a yellow phenotype.

Monohybrid inheritance

• Monohybrid involves one gene, with two different alleles.

• One trait is dominant and the other is recessive. It is important to note that the traits are dominant or recessive not the actually alleles.

• Pure breeding parents – homozygous for that gene

• Individuals with two different alleles = hybrids – heterozygous for that gene.

Monohybrid inheritance

• Complete dominance

The law of segregation

• Inherited characteristics are controlled by genes that occur in pairs.

• The alleles of each gene will separate during meiosis so that each gamete only have one allele of each gene

The law of independent assortment

• Each gene pair is inherited independent of other gene pairs during gamete formation.

• Alleles of one gene assort independently of the alleles of another gene during meiosis.

Identifying recessive alleles

• If the parents do not express the recessive trait, they must be homozygous. In such a case.– The first appearance of the recessive trait

within a family is usually in the F1 generation.– 25% of this F1 will express the trait– Both males and females can express the trait

unless it is a recessive sex-linked gene.

• If one parent does express the recessive trait– That parent must be homozygous– The trait will not be expressed in every

generation– Both males and females can express the trait

unless it is a recessive sex-linked gene

• Identifying dominant traits– If the trait is dominant, at least some offspring in all

generations will show the trait– The trait is passed from the affected parent to at least

50% of the F1 generation. If at least one parent is homozygous for the dominant trait, then all offspring will show it. If both parents are heterozygous and the other is heterozygous for the recessive trait then 50% of offspring will show it.

– Any parents that does not express the trait does not transmit it to any of the F1.

– Both males and females can express and transmit the trait.

Test crosses

• Are a way of determining whether an organism is homozygous or heterozygous

• Test crosses are used to determine an organism’s genotype.

• We test cross with individuals that are know to be homozygous recessive.

• By examining the offspring we are able to identify the genotype of the individual.

Test crosses

• Can be used to determine an unknown genotype.– In a test cross an individual showing the

dominant phenotype but of unknown genotype is crossed with a homozygous recessive genotype.

• Consider the following examples of crosses between Aa X aa, and AA x aa

Test crosses

• If the unknown genotype is Aa, then at least half of the offspring could be expected to show the recessive character. Genotype ratio Aa:aa = 1:1.

• If unknown genotype is AA, then none of the offspring would show the recessive character. All of the offspring would show the dominant feature. Genotype: all Aa.

A a

a Aa aa

a Aa aa

A A

a Aa Aa

a Aa Aa

Dihybrid inheritance

Relationships and Dominance

Co-dominance and incomplete (partial) dominance

Co-dominance and incomplete (partial) dominance

• Co-dominance refers to a pattern of inheritance in which heterozygous individuals have a phenotype different form the phenotypes of both of the homologous individuals, and the effects of the two different alleles are both evident in this phenotype.

• Inheritance of ABO blood types in humans is an example of this.

• One gene locus controls the production of certain antigens present on the surface of red blood cells. This gene has three alleles.

Blood gene alleles

Allele Expression

IA A type antigens produced

IB B type antigens produced

i Neither A nor B produced

• Notice that when dealing with co-dominant of incompletely dominant inheritance, alleles are shown as capital letters with a superscript/subscript letter or number distinguishing the alleles.

Co-dominance and incomplete dominance

• As there are three possible alleles at this locus, an individual may have one of six genotypes.

• The alleles IA and IB are codominant and so are both expressed if present.

• The six possible genotypes can produce four possible phenotypes

Genotype Expression Phenotype

IAIA A type antigens produced

A type blood

IAiA type antigens

producedA type blood

IAIB

A type antigens produced and B type antigens

produced

AB type blood

IBIB B type antigens produced

B type blood

IBiB type antigens

producedB type blood

iiNeither A nor B

producedO type blood

For your consideration

• Consider a cross between two parents IBi and IAi.

• These parents can produce offspring with all four possible phenotypes; each is equally likely.

• Again the gamete probabilities are multiplied to find the probability of each genotype in the offspring.

Another example

• Another example of co-dominance occurs in coat colour in some cattle and horses.

• The offspring of reddish animal and a white animal will often have the coat colour called roan, a pinkish tan.

• This pinkish appearance is due to the presence of both white and red hairs.

• Both of the alleles are expressed in different individual hairs.

• What offspring might you expect if a roan cow was crossed with a red stallion?

Incomplete or Partial dominance

• Incomplete dominance refers to a pattern of inheritance where heterozygote are different from both homozygotes, but where the separate effects of the two alleles cannot be seen in the phenotype.

• Examples of incomplete dominance occur in the genes affecting flower colour in many plants species.

• One such plant species is Japanese four o’clocks. • One gene determines flower colour. It has two alleles: R1 and R2. • Individual plants have one of three possible genotypes.• These genotypes give rise to three distinct phenotypes.

Genotype Phenotype

R1R1 Red flowers

R2R2 White flowers

R1R2 Pink flowers.

Multiple alleles

Multiple Alleles• As you saw with the example of human ABO blood groups, genes

can and often do, have more than two possible alleles.• An example of multiple alleles at a single gene locus occurs in the

inheritance of coat colour in dogs.• Early wild type dogs were nearly always agouti in colour, with

genotype AA.• During centuries of domestication a number of alternative alleles of

this gene have arisen by mutation. • Amongst dogs today there are five recognized alleles at this one

locus.• The following table shows these five phenotypes in order of

dominance, and the genotypes which give rise to each phenotype.• Remember each individual only inherits 2 alleles, regardless of

how many different alleles exist.

Multiple Alleles

Phenotype Allele Symbol Genotypes

Solid black AS AS AS , AS AY, AS A, AS asa, As at

Dominant yellow AY AY AY, AY A, AY asa, AY at

Agouti A A A, A asa, A at

Saddle pattern asa asa asa, asa at

Tan pattern at at at

• The five alleles at this locus means there are 15 different genotypes and 5 different phenotypes.

• The table on the next page shows the number of possible genotypes for a locus with multiple alleles.

Multiple Alleles

Number of alleles

Number of genotypes

Calculation

2 3 2+1

3 6 3+2+1

4 10 4+3+2+1

5 15 5+4+3+2+1

6 21 6+5+4+3+2+1

n n+(n-1)+(n-2)+………..1

• The number of phenotypes is a bit more complicated. If the alleles in the series all show complete dominance then the number of possible phenotypes is equal to the number of possible alleles.

• If however, some of the alleles are co-dominant there can be more phenotypes, e.g. the human ABO blood group has 3 alleles, 6 genotypes and 4 phenotypes.

Lethal genotypes

• Some dominant traits are lethal in homozygous individuals. An example of this occurs in a gene which determines coat colour in rats.

• A mutant allele (Y) produces yellow pigmentation in heterozygous (Yy) rats. The alternative allele (y) produced normal pigmentation in homozygous rats.

• A zygote which is homozygous (YY) for the yellow allele fails to develop and hence is never born.

• Consider a cross between two yellow rats. (Yy X Yy).• Litter sizes will smaller than average, as ¼ of offspring

are not born.• Those which are born are yellow or normal pigmented in

the ratio of 2:1

Lethal genotypes

Gametes ½ Y ½ Y

½ Y ¼ YY (lethal) ¼ Yy (yellow)

½ y ¼ Yy (yellow) ¼ yy (normal)

Polygenic Inheritance

Polygenic inheritance• Many aspects of phenotype are influenced by many genes.• These traits are known as polygenic.• The greater the number of genes influencing a phenotype the great

the variation there will be in the trait in a population of organisms.• Traits influenced by a single gene generally have only two of three

possible phenotypes.• Populations of organisms show discontinuous variation for these

traits.• Consider the gene for the Rhesus blood factor in humans.• There are two possible phenotypes, Rh+ and Rh-, and each person

has one of these two.• Data for Rhesus blood factor in a population for the general

population would look like this.• Each person is Rh+ or Rh-, there is nothing in between.

Polygenic inheritance

• Feather colour in budgerigars is influenced by three genes.

• Variations in these three genes produce seven feather colour phenotypes in budgies.

• Each colour is distinguishable from the others, so the budgies are still said to show discontinuous variation.

• The feather colour data for a group of domestic budgerigars might look something like this.

Polygenic inheritance

• When a large number of genes influence an aspect of phenotype a population will show continuous variation.

• Individuals phenotypes will be impossible to distinguish.

• There are many genes which influence height. The interaction of all of these genes produces continuous variation in a population of organisms.

• Similar patterns of discontinuous variation are found in other polygenic traits such as skin colour, weight and seed size.

Pedigree Analysis

Pedigree analysis• A pedigree can be drawn to show the inheritance of a trait in a

family or families.• Pedigrees show only phenotypes but often genotypes can be

worked out from them.• If you are asked to determine whether a trait is dominant or

recessive, sex linked or autosomal you need to model each possibility to see if it is possible.

• Too often students assume the most common trait in a pedigree is dominant, or that a lot of affected males indicates that the trait must be sex linked.

• The only way around these mistakes is to make and test assumptions about the inheritance of the trait.

• The table on the next page shows a number of pedigrees and what can be deduced from them.

• In all of these examples ‘A’ represents the dominant phenotype, and ‘a’ represents the recessive phenotype.

Pedigree Analysis

• Male

• Female

Autosomal Recessive Inheritance

• Two copies of the allele must be present for an individual to have the trait

• Both males and females can be affected• Characteristic can ‘skip’ a gneration or generations• Affected individual can have unaffected parents, as both

may be heterozygotes (carriers), so they do not actually show the trait

• If both parents have the trait, then all the offspring will have the trait.

• Over many generations approximately equal numbers of males and females will be affected.

Autosomal dominant inheritance

• One copy of an allele is sufficient for expression of a trait• The allele must be expressed in every generation for the

trait to continue (once it disappears from a branch of the pedigree it does not reappear, unless it is reintroduced by a person from outside the family)

• Males and females are affected• All affected individuals have at least one parent with the

trait• Transmission can be from either parent to either sons or

daughters• Over many generations approximately equal numbers of

males and females will be affected.

Sex (X) linked recessive inheritance

• Males are XY therefor an X-linked allele present will be expressed.

• Females are XX and will need to carry a copy of the gene on each of their X chromosome for the characteristic to be expressed.

• A mother with the trait passes it on to all of her sons.• All daughters of the fathers with the trait are carriers,

therefore will not show the trait although their sons may have the trait.

• Fathers with the trait cannot pass the characteristic to their sons

• If both parents show the trait the children will have the trait

• More males than females show the trait

Sex (X) linked dominant inheritance

• All daughters of affected fathers have the trait

• Mothers may pass it onto sons and daughters

• The trait does not ‘skip’ generations to reappear in later generations.

• Over large numbers of generations more females than males are affected.

Pedigree Dominant/Recessive? X-Linked (Sex-linked) or Autosomal

Two parents who do not have the trait have produced children who do.

The trait must be recessive.

It is recessive. If it is X-linked then the daughter must be XaXa.

For her to have this genotype her father would have to be XaY. He is not!. The trait cannot be recessive and X-linked. The trait must be autosomal.

Two parents who have the trait have produced children who do not. The trait must be dominant.

It is dominant. If it was X-linked parental genotypes can be XAXa and XAY. The boys are XaY and the girls could be XAXA or XAXa.

X-linkage is possible, though not proved. This pedigree is also consistent with autosomal dominant inheritance

Two parents who do not have the trait have produce children who do. The trait must be recessive.

It is recessive. If it is X-linked then the sons are XaY and the daughter is XA_. These genotypes are possible if the parents are XAXa and XAY. X-linkage is possible, though not proved. If the sample size was very large and females were always unaffected and males often affected, you could infer X-linkage.

One parent and one child have the trait. You cannot conclude dominance from this. If it is recessive, the parents are: male Aa and female aa. If it is dominant: male aa and female Aa. Both possibilities are consistent with the information in the pedigree.

If it is X-linked and recessive then the parental genotypes are XAY and XaXa. All of the sons would be affected. One is not affected so the trait cannot be X-linked and recessive. If it was X-linked and dominant the parents would be XaY, XaXa and XAY. This is possible given what we know.