4.3 Theoretical Genetics

Topic 4 Genetics

Theoretical Genetics 4.3.1 Define genotype, phenotype, dominant allele,

recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

Genotype: the alleles of an organism. Phenotype: the characteristics of an organism. Dominant allele: an allele that has the same effect on the

phenotype whether it is present in the homozygous or heterozygous state.

Recessive allele: an allele that only has an effect on the phenotype when present in the homozygous state.

Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote.

Theoretical Genetics Locus: the particular position on homologous

chromosomes of a gene. Homozygous: having two identical alleles of a gene. Heterozygous: having two different alleles of a gene. Carrier: an individual that has one copy of a recessive

allele that causes a genetic disease in individuals that are homozygous for this allele.

Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.

Theoretical Genetics 4.3.3 State that some genes have more than two alleles

(multiple alleles).

4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.

4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

Theoretical Genetics 4.3.7 Define sex linkage.

4.3.8 Describe the inheritance of colour blindness and hemophilia as examples of sex linkage.

Both colour blindness and hemophilia are produced by a recessive sex-linked allele on the X chromosome. Xb and Xh is the notation for the alleles concerned. The corresponding dominant alleles are XB and XH.

4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles.

Theoretical Genetics 4.3.11 Predict the genotypic and phenotypic ratios of

offspring of monohybrid crosses involving any of the above patterns of inheritance.

Aim 8: Statisticians are convinced that Mendel’s results are too close to exact ratios to be genuine. We shall never know how this came about, but it offers an opportunity to discuss the need for scientists to be truthful about their results, whether it is right to discard results that do not fit a theory as Louis Pasteur is known to have done, and the danger of publishing results only when they show statistically significant differences.

Theoretical Genetics TOK: Reasons for Mendel’s theories not being accepted

by the scientific community for a long time could be considered. Other cases of paradigm shifts taking a long time to be accepted could be considered. Ways in which individual scientists are most likely to be able to convince the scientific community could be considered, and also the need always to consider the evidence rather than the views of individual scientists, however distinguished.

Theoretical Genetics 4.3.12 Deduce the genotypes and phenotypes of

individuals in pedigree charts. For dominant and recessive alleles, upper-case and lower-

case letters, respectively, should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case. For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. For example, red and white codominant flower colours should be represented as CR and Cw, respectively. For sickle-cell anemia, HbA is normal and Hbs is sickle cell.

Theoretical Genetics Aim 8: There are many social issues in families in which

there is a genetic disease, including decisions for carriers about whether to have children, personal feelings for those who have inherited or passed on alleles for the disease, and potential problems in finding partners, employment and health or life insurance. There are ethical questions about whether personal details about genes should be disclosed to insurance companies or employers. Decisions may have to be made about whether or not to have screening. These are particularly acute in the case of Huntington disease.

Terminology There are some terms you need to be able to define:

Genotype Phenotype Dominant allele Recessive allele Codominant allele Locus Homozygous Heterozygous Carrier Test cross

Punnett Grids A punnett grid is a way of finding the

expected ratio of the offspring, given certain parental phenotypes.

Punnett grid can be constructed looking at one or two characteristics: One chacteristic – a monohybrid

cross Two characteristics – a dihybrid cross

Standard level only need to do Monohybrid crosses.

To the right is a simple punnett grid for sex determination in humans.

Ref: Biology, Weem

Pedigrees A pedigree chart shows the members of a family and how they are

related to each other. It can also show ancestral history of a group of related individuals. Pedigree charts can also be used to study the inheritance of a

characteristic. By convention:

Circles represent females and squares represent males Shaded circles or shaded squares represent affected individuals;

unshaded circles or squares represent unaffected individuals. Two parents are linked by a horizontal line joining a circle and a

square. Vertical lines run down from parents to children. The children in one family are linked by a horizontal line above them.

Pedigrees A pedigree showing the inheritance of haemophillia in Queen Victoria’s family

Ref: Advance Biology, Kent

Multiple Alleles Alleles always occur in pairs because chromosomes are in

pairs and so the alleles occupy the pair of gene loci on homologous chromosomes.

Usually within a population there are many different alleles of a gene and this is called Multiple alleles.

But any one individual can only have two of these alleles because it only has one pair of loci for any given gene.

Codominance Codominance means that both alleles have an effect on

the phenotype. ie: the phenotype is some form of mixture of the two

characteristics. Neither gene is dominant or recessive.

The ABO Blood Groups The ABO blood grouping is an example of multiple

alleles and codominance. To determine you blood type, there are three alleles:

IA, IB and i.

This is an unusual case because they also show codominance: Alleles IA and IB are codominant.

This results in four different phenotypes.

The ABO Blood Groups

Phenotype or blood group

Genotypes

A IAIA or IAi

B IBIB or IBi

AB IA IB

O ii

Sex Determination Two chromosomes determine the gender of a child (whether it is male

or female). These are called the sex chromosomes. The X chromosome is relatively large and carries many genes. The Y chromosome is much smaller and carries only a few genes. If two X chromosomes are present in a human embryo, it develops

into a girl. If one X and one Y chromosome are present in a human embryo, it

develops into a boy. When women produce gametes, they pass on one X chromosome in

each egg. When men produce gametes, they pass on either one X or one Y

chromosome in the sperm so the gender of the child depends on whether the sperm that fertilises the egg is carrying an X or a Y chromosome.

Sex Determination

This also shows that there is a 50:50 chance of a child being a boy or a girl.

Ref: Biology for the IB Diploma, Allott

Sex Linkage Genes that are located on one of the sex chromosomes

are said to be sex-linked. Because the X chromosome is much larger than the Y

chromosome, most sex-linked genes are located on the X chromosome.

As well as carrying genes for sex characteristics, the X chromosome also carries genes for non-sexual characteristics.

Two well studied sex-linked examples are: Haemophilia Colour blindness

Sex-linkage in Males & Females In respect to sex-linked genes human females can be:

Homozygous XNXN or XnXn

Heterozygous XNXn

Females who are heterozygous for X-linked recessive alleles are said called Carriers.

Males only have one X chromosome so their inheritance is a little different. They can be: Normal XNY Affected XnY

Haemophilia Haemophilia is a condition in which the blood does not

clot normally. It normally results in excessive bleeding both internally

and externally. The condition is due to the lack of one or more clotting

factors, the most common is a lack of Factor VIII. Individuals can lead a normal life with regular injections

of factor VIII. Haemophilia is a sex-linked characteristic caused by a

recessive allele carried on the X chromosome.

Haemophilia Females can be either homozygous dominant or

heterozygous for haemophilia. XHXH normal XHXh carrier

They cannont be homozygous recessive because the haemophilia allele is homozygous lethal. Ie: XhXh does not happen because they die.

Males can be either normal or affected: XHY normal XhY affected - haemophiliacs

Ref: Biology for the IB Diploma, Allott

Colour Blindness Another X-linked condition is colour blindness. Females:

XBXB Normal vision (homozygous dominant)

XBXb Normal vision – carrier (heterozygous) XbXb Colour Blind (homozygous

recessive) Males

XBY Normal vision XbY Colour Blind

The most common form of colour blindness is red-green colour blindness.

6-8% of Caucasian males are red-green colour blind while only 0.5% of women have this condition

Using Punnett Grids In choosing symbols for alleles in Punnett grids, these

rules are usually followed: Dominant and recessive alleles or genes.

One letter of the alphabet is chosen. The dominant allele is represented by the upper-case letter and the recessive allele by the lower-case letter.

e.g. A and a Codominant alleles.

One letter of the alphabet is chosen. This letter and a superscript letter represent each allele

e.g. Cw and Cr

Sex-linked dominant and recessive alleles The letter X is used to symbolise the X chromosome. Each allele is

shown superscripted. e.g. XH and Xh

Using Pedigree Charts You can use pedigree charts to study the inheritance of a

characteristic. Pedigree charts can also be used to determine if:

The characteristic is dominant or recessive. The characteristic is sex-linked or not.

Look at the examples on the handout and try the questions.

IBO Guide: 4.3.1 Define genotype, phenotype, dominant allele,

recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

4.3.3 State that some genes have more than two alleles (multiple alleles).

4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.

4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

IBO Guide:4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.4.3.7 Define sex linkage.

4.3.8 Describe the inheritance of colour blindness and hemophilia as examples of sex linkage.

4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles.

IBO Guide: 4.3.11 Predict the genotypic and phenotypic ratios of

offspring of monohybrid crosses involving any of the above patterns of inheritance.

Aim 8: Statisticians are convinced that Mendel’s results are too close to exact ratios to be genuine. We shall never know how this came about, but it offers an opportunity to discuss the need for scientists to be truthful about their results, whether it is right to discard results that do not fit a theory as Louis Pasteur is known to have done, and the danger of publishing results only when they show statistically significant differences.

IBO Guide: 4.3.12 Deduce the genotypes and phenotypes of

individuals in pedigree charts. For dominant and recessive alleles, upper-case and lower-

case letters, respectively, should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case. For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. For example, red and white codominant flower colours should be represented as CR and Cw, respectively. For sickle-cell anemia, HbA is normal and Hbs is sickle cell.

IBO Guide: Aim 8: There are many social issues in families in which

there is a genetic disease, including decisions for carriers about whether to have children, personal feelings for those who have inherited or passed on alleles for the disease, and potential problems in finding partners, employment and health or life insurance. There are ethical questions about whether personal details about genes should be disclosed to insurance companies or employers. Decisions may have to be made about whether or not to have screening. These are particularly acute in the case of Huntington disease.