Lecture 22 Extending Mendelian Genetics PDF

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Lecture 22 Extending Mendelian Genetics Learning Outcomes  Recognise and explain the molecular basis of different dominance relationships between alleles within a gene  Distinguish between incomplete dominance, co-dominance, multiple alleles, epistasis, polygenic inheritance, and sex-linked genes. Recognise cases of each of these and solve problems based on them  Explain why in a population having multiple alleles for a single gene, most organisms in that population only have two alleles  Apply the concept of epistasis to genetic crosses between mutants in a metabolic pathway  Explain why, despite apparent ‘blending’ in phenotypes with quantitative traits, Mendel’s laws are still obeyed So far we have only considered complete dominance, in which phenotypes of heterozygous and homozygous dominant individuals are the same. However, in some cases complete dominance is not seen and classical Mendelian ratios (e.g. 3:1 and 9:3:3:1) are not observed. Modified Mendelian ratios can alert us to interactions between gene products. Sometimes this involved interaction between two different alleles of a single gene in a heterozygote, sometimes it involves products of different genes that affect the same phenotype Mutations about Dominance The following statements are NOT true.  Dominant alleles are more likely to be inherited than recessive alleles  Dominant alleles are more common in populations than recessive alleles  Dominant alleles regulate the expression of recessive alleles  Mutations are always recessive  Recessive alleles are always deleterious  Natural selection causes beneficial traits to become dominant over time Single-gene extensions of Mendelian Genetics Dominance relationships Co-dominance and incomplete dominance are recognised by the phenotypes of heterozygotes, compared to those of the parents (or of homozygous offspring). Multiple alleles So far we have looked at cases where only two different alleles for any gene exist, but this is not the case for many genes. However, each diploid individual can only possess, at most, two different alleles of any given gene. This of course is because diploid individuals carry two copies of each gene, one on each homologous chromosome. Multiple-gene extensions of Mendelian Genetics

Epistasis Note that epistasis is not dominance. Dominance occurs between the alleles of a single gene. Epistasis occurs between genes.

Notice in Campbell’s discussion of coat colour in dogs, even though these two genes affect the same phenotypic character, they assort independently. Thus, in the F2 of a dihybrid cross (assuming independent assortment), the following genotype pattern will exist: 9B_C_: 3B_cc: 3bbC_: 1bbcc

Since we know how the C and c alleles affect the phenotype of the B gene, we can apply these conditions to the above ratio and predict 9 black:3 brown:4 albino which is what is observed. The BBcc, Bbcc, bBcc and bbcc genotypic classes are grouped together, they are all albino because they lack a C allele required for pigment synthesis. A clue to help you identify epistasis is an altered Mendelian ratio, such 12:3:1, 9:3:4, 9:6:1, etc. Polygenic Inheritance Mendelian analysis depends on the ability to classify individuals into discrete phenotypic categories (e.g. smooth or wrinkled seeds, white or purple flowers). These can be described as qualitative differences between individuals. However many characteristics observed in nature do not vary in this way. Characteristics such as size, shape and behaviour, often vary more or less continuously in a population over a range of values. This is referred to as quantitative variation because it must be described with a measurement. Quantitative characters do not at first appear to be inherited in a Mendelian fashion. When a very tall wheat plant is crossed with a short one, the progeny tend to be intermediate in height. This is the type of observation that led to the “blending” model of inheritance that preceded Mendel’s work. However it is not an example of incomplete dominance. When the F1 are selfed, the F2 generation do not fall nicely into a 3:1 ratio of tall to short, or even a 1:2:1 ratio of tall, intermediate and short plants. Instead, they tend to vary continuously in height from one parental extreme to the other in a bell-shaped or normal distribution. This behaviour does not reflect a violation of Mendel’s laws. It can be explained by proposing that many different genes control the phenotype. This is referred to as polygenic inheritance. Sex-linked Genes There are other instances in which simple Mendelian ratios are not observed. Any gene located on a sex chromosome (as opposed to being on an autosome) is said to be sex-linked. As (human) X chromosomes are large, they carry many genes that are not found on Y chromosomes, so almost all human sex-linked genes are in fact found on the X chromosome, and you can assume that a human gene called sex-linked is in fact X-linked. Because of the unique pattern of inheritance of sex-linked genes, a specific type notation must be used to represent them when completing Punnett squares. Use the usual XX and XY to represent the sexes of the individuals, and use superscript symbols to represent the alleles of the sex-linked gene. For example, XHXh is a female carrier of haemophilia, XHY is a normal male and XhY is male with haemophilia. Since the X and Y chromosomes segregate during meiosis like the 22 pairs of homologous chromosomes, you can complete Punnett squares in the same way. Note that if you did not use this notation, you’d give the male an extra allele that he should not have...