7.1 Inheritance

Question 1

Genotype

Answer 1

The genotype is the genetic constitution of an organism.

Question 2

Phenotype

Answer 2

The phenotype is the expression of the genotype and its
interaction with the environment.

Question 3

Alleles

Answer 3

Random mutation can result in new alleles of a gene.
Genetic diversity is the number of different alleles of genes in a
population - there may be many alleles of a single gene in a population.
An individual inherits alleles from their parent or parents.
A change in the allele frequency of a population is
evolution.
Alleles may be dominant, recessive or codominant.
In a diploid organism, the alleles at a specific locus may be either
homozygous or heterozygous.

Question 4

Genetic diagrams

Answer 4

You have to use fully labelled genetic diagrams (Punnett squares) to interpret, or predict, the results of:
* monohybrid and dihybrid crosses involving dominant, recessive and codominant alleles
* crosses involving sex-linkage, autosomal linkage, multiple alleles and epistasis.

Question 5

Chi-squared

Answer 5

Genetic diagrams give you expected phenotypic ratios.

You have to use the chi-squared test to compare the ‘goodness of fit’ of observed phenotypic ratios with expected phenotypic ratios. If chi squared results in a p value of 0.05 or lower, then there is a significant difference between the observed phenotypic ratios and the expected phenotypic ratio, meaning the assumptions made in producing the Punnet square were incorrect.

Question 6

Codominance

Answer 6

Two different alleles at the same locus are both expressed in the phenotype.

Question 7

Autosomal linkage

Answer 7

If two genes have their loci on the same chromosome, they are said to be autosomally linked (an autosome is any chromosome that is not a sex chromosome).

A dihybrid cross genetic diagram assumes the gametes have been produced by normal meiosis, including independent assortment and crossing over.

The alleles of two genes that are autosomally linked are not separated by independent assortment, although they may be separated by crossing over (the further apart the two loci are from each other on the chromosome, the more likely they are to be separated by crossing over) therefore the observed ratio will be not a a ‘good fit’ to the expected ratio.

Question 8

Sex linkage

Answer 8

In sexually reproducing species, the biological sex of offspring is determined by the inheritance of sex chromosomes. While the autosomes from each parent can form homologous pairs, some sex chromosomes can not, usually because one is shorter than the other.

In mammals (and many other taxa) females inherit XX and males inherit XY (but be prepared to find exceptions - in birds for example males are ZZ, and females are ZW). In female mammals, the sex chromosomes are homologous, but in mammalian males, they are not.

Any gene that is located on a sex chromosome is described as sex linked. Most sex-linked genes are found on the X chromosome as it is larger and so carries more genes than the Y. As a result XX females can be carriers of a recessive allele, but XY males must express recessive alleles on the X chromosome. This makes X-linked recessive disorders (e.g., haemophilia, colour blindness) more common in males, whereas in females (XX) a recessive allele must be homozygous to be expressed.

Example:

Haemophilia: Xᴴ = normal, Xʰ = haemophilia allele

Female carrier: XᴴXʰ

Affected male: XʰY

Question 9

Epistasis

Answer 9

Epistasis occurs when one gene affects the expression of another gene at a different locus. The gene that masks the effect of the other is called the epistatic gene. In recessive epistasis two recessive alleles at one locus mask the expression of the second gene, whereas in dominant epistasis a dominant allele at one gene locus masks the effect of the other gene.

Imagine a metabolic pathway that converts molecule 1 into first molecule 2 and then molecule 3. This pathway has two enzymes, A and B - the product from A being the substrate for B (so A turns 1 into 2, and B turns 2 into 3). If the cell genotype is AaBb it can complete the pathway, but if it is Aabb it cannot, and the pathways stops at molecule 2.

Now, suppose the genotype is either aaBB or aaBb or aabb - all of these fail to start the pathway, so in all three genotypes the molecule remains unchanged as 1. This is recessive epistasis - the homozygous recessive form of the gene for enzyme A completely masks the genotype of enzyme B - it doesn’t matter if the second enzyme genotype is BB, Bb or bb - it will never be expressed in the phenotype if enzyme A is aa.