review a gene represents the genetic material on a chromosome that contains the instructions for...

Review

• A gene represents the genetic material on a chromosome that contains the instructions for creating a trait

• An allele is one of several varieties of a gene

• A locus is the location of a gene on a chromosome

• Homologous chromosomes refer to a pair of chromosomes that contains the same genetic information, gene for gene

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

Gregor Mendel

• The father of modern genetics is Gregor Mendel

• He bred pea plants in order to study patterns of inheritance

• He studied clear-cut traits, over ten thousand plants, and applied statistical analysis

• Mendel’s theory of genetics is one of particulate inheritance, in which inherited characteristics are carried by discrete units called genes

Mendel used the scientific approach to identify two laws of inheritance

• True-breeding parents in a genetic cross are called the P (parental) generation; their offspring are called the F1 (first filial) generation

• If the F1 population is crossed, their offspring are called the F2 (second filial) generation


Mendel’s Experimental, Quantitative Approach

• Advantages of pea plants for genetic study:

– There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits

– Mating of plants can be controlled

– Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

– Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another


Fig. 14-2

TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-ationoffspring(F1)

5

Fig. 14-2a

StamensCarpel

Parentalgeneration(P)

TECHNIQUE

1

2

3

4

Fig. 14-2b

Firstfilialgener-ationoffspring(F1)

RESULTS

5

• Mendel chose to track only those characters that varied in an either-or manner

• He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)


• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced


The Law of Segregation

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation


Fig. 14-3-1

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

Fig. 14-3-2

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

Fig. 14-3-3

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

• What Mendel called a “heritable factor” is what we now call a gene


Four Concepts of Mendel’s Model

• Alternative versions of genes cause variations in inherited characteristics among offspring

– The gene for flower color in pea plants comes in two versions: white and purple (alleles)

• For each character, every organism inherits one allele from each parent

Four Concepts of Mendel’s Model

• If the two alleles are different, then the dominant allele will be fully expressed in the offspring, whereas recessive allele will have no noticeable effect on the offspring

• The two alleles for each character separate during gamete production

Segregation vs. Dominance

• Law of Segregation

– During the formation of gametes, the two traits carried by each parent separate

• Law of Dominance

– When two organisms for two opposing traits are crossed, the offspring will be hybrid, only exhibiting the dominant trait

– The trait that remains hidden is the recessive trait

Punnett Square

• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

Fig. 14-5-1

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP

P

pp

p

Fig. 14-5-2

P Generation


Gametes:


P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/21/2

Fig. 14-5-3

P Generation


Gametes:


P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/21/2

F2 Generation

Sperm

Eggs

P

PPP Pp

p

pPp pp

3 1

Genetic Terms

• Homozygous organisms have two of the same alleles for a particular trait

– Dominant alleles for a trait are capitalized and the recessive allele is not capitalized

– Individuals could be homozygous for the dominant trait (RR) or the recessive trait (rr)

• Heterozygous organisms have two different alleles for a trait (Rr)

Phenotype vs. Genotype

• Phenotype refers to an organism’s expressed physical traits (round seed)

• Genotype refers to an organisms genetic makeup (RR or Rr)

Fig. 14-6

Phenotype

Purple

Purple3

Purple

Genotype

1 White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

Testcross

• A testcross is done to determine if an individual showing a dominant trait is homozygous or heterozygous (genotype)

– The individual in question (B/_) is crossed with a homozygous recessive individual (b/b)

– If the individual being tested is in fact homozygous dominant, then all offspring of the testcross will be B/b and will show the dominant trait

– No offspring can show recessive traits; if so, the parent is hybrid

Monohybrid vs. Dihybrid

• A monohybrid cross is a cross involving the study of one character (flower color)

• A dihybrid cross is a cross intended to study two characters (flower color and seed shape); can also show whether two different traits come as a package or independently (Law of Independent Assortment)

– Genes located near each other on the same chromosome tend to be inherited together

Multiplication Rule

• To find the probability of two independent events happening, multiply the chance of one happening by the chance that the other will happen

• For example, the chance of a couple having two boys depends on two independent events:

– The chance of the first child being a boy is ½

– The chance of the next child being a boy is ½

– The chance that the couple will have two boys is therefore ½ x ½ = ¼

Addition Rule

• When more than one arrangement of events producing the specified outcome is possible, the possibilities for each outcome are added together

• For example, if a couple is planning on having two children, what are the chances of having one boy and one girl (in either order)?

– Probability of boy then girl is ½ x ½ = ¼

– Probability of girl then boy is ½ x ½ = ¼

– Therefore, the probability of having one boy and one girl is ¼ + ¼ = ½

Complete Dominance

• Dominance in which the heterozygote and the homozygote for the dominant allele are indistinguishable

– A Yy yellow seed is just as yellow as a YY yellow seed

Codominance

• Codominance occurs when two alleles are dominant and affect the phenotype in two different but equal ways

– Example: blood type AB

– Both alleles (A and B) are completely expressed

Incomplete Dominance

• A type of dominance in which the F1 hybrids have an appearance that is in between that of the two parents

– Example: straight hair (H) and curly hair (H1)

• child born with wavy hair (HH1)

– Example: red flower (R) and white flower (R1)

• Offspring flower is pink RR1

Multiple Alleles

• Occur when a gene has more than two alleles

• Can be seen in human blood types

– Ia , Ib , and i

• The three alleles combine to form different blood types

Pleiotropy

• Property of a gene that causes it to have multiple phenotypic effects

– Example: sickle cell disease is caused by an allele that incorrectly codes for hemoglobin

• As a result, RBCs do not flow through capillaries freely, oxygen is not adequately delivered, the heart, lungs, kidneys, brain and other organs are damaged, etc.

– Example: frizzle trait in birds

Epistasis • Two separate genes control one trait, but one gene masks

the expression of the other gene

– The gene that masks the other is epistatic

– Example:

• A gene related to melanin production has two alleles, C which calls for melanin and c which does not call for melanin

• Therefore, c/c results in an albino animal

• The second gene distributes melanin, with B distributing it completely and b distributing a moderate amount

• However, neither B nor b can distribute melanin since c/c calls for no melanin

Polygenic Inheritance

• Many traits are not expressed in just two or three varieties

• The heights of humans, for example, are not just short or tall; they are displayed as a continuous variation from very short to very tall

• Polygenic inheritance is the interaction of many genes to shape a single phenotype

• Two parents who are short carry more genes for shortness than tallness, but they can have a child who inherits mostly genes for tallness

Polygenic Inheritance

• The child will be taller than the parents

• This wide variation in genotypes always results in a bell-shaped curve in an entire population

Pedigree

• A pedigree is a diagram that shows the relationship between parents and offspring across two or more generations

– Circles represent women; squares represent men

– White open circles or squares indicate that the individual did not or does not express a particular trait, whereas black indicates that the individual expresses or expressed the trait

Pedigree

• Through the patterns they reveal, pedigrees can help determine the genome of individuals that comprise them; pedigrees can also predict the genome of future offspring

Fig. 14-15b

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Wwww ww

ww

wwWw

Ww

wwWW

Wwor

Fig. 14-15c

Attached earlobe

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

orFF

Ff

Recessively Inherited Disorders

• Require two copies of the defective gene for the disorder to be expressed

– Cystic fibrosis: caused by a mutation in an allele that codes for a cell membrane protein that helps with the transport of chloride ions in and out of cells

• The resulting high extracellular levels of chloride cause mucus to be thicker and stickier, leading to organ malfunction and recurrent bacterial infections


• Tay-Sachs: caused by an allele that codes for a dysfunctional enzyme, which is unable to break down certain lipids in the brain

– As these lipids accumulate in brain cells, the child suffers from blindness, seizures, and degeneration of brain function, leading to death


• Sickle Cell Disease: caused by an allele that codes for a mutant hemoglobin molecule that forms long rods when the oxygen levels in the blood are low

– These long rods cause the red blood cell to sickle, clogging blood vessels and leading to pain, organ damage and even paralysis

Lethal Dominant Alleles

• Require only one copy of the allele in order for the disorder to be expressed

• Usually, only late-acting lethal alleles are passed on

• Huntington’s disease is caused by a lethal dominant allele; it is a degenerative disease of the nervous system, which usually doesn’t affect the individual until 40 years of age

Genetic Testing

• May be used on a fetus to detect certain genetic disorders

– Amniocentesis occurs when the physician removes amniotic fluid from around the fetus; the fluid can be utilized to detect some genetic disorders and the cells in the fluid can be cultured for a karyotype

– Chlorionic villus sampling involves using a narrow tube inserted through the cervix to suction out a tiny sample of placenta containing only fetal cells; a karyotype can immediately be developed

• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

• Probability in an F1 monohybrid cross can be determined using the multiplication rule

• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele

The Multiplication and Addition Rules Applied to Monohybrid Crosses


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review a gene represents the genetic material on a chromosome that contains the instructions for...

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