Lethal alleles

1. Lethal alleles

2. What is a lethal allele? • Lethal alleles occur when a mutation results in an allele that produces a non-functional version of an essential protein. • If an individual inherits a lethal combination of mutated alleles, it will die before or shortly after birth.

3. Huntington’s chorea • http://www.youtube.com/watch?v=PCVO9c9q4tE • http://www.youtube.com/watch?v=MRZoM5L5dak

4. Pedigree chart for Huntington’s disease

5. Genetic cross for Huntington’s

6. Abnormal ratios in genetic crosses • It is probably true that in genetic studies more has been learned from abnormal ratios than that of normal ones.

7. Consider, for example, the inheritance of fur colour in mice. • In mice, the allele for yellow fur (Y) is dominant to the allele for grey fur (y) • What ratios would you expect if a pair of heterozygous yellow mice are mated?

8. We naturally expect three quarters of the mice to be yellow and a quarter to be grey. • However, this is not the result we actually observe when this genetic cross is carried out. • The observed results are 2/3 of the offspring are yellow and 1/3 are grey.

9. So what is going on? • A possible explanation is that mice that are homozygous for the yellow allele (YY) die before birth.

10. Or as a punnet square Yy Y y Y YY Yy dead yellow alive Yy y Yyyy yellow grey alive alive

11. What is the evidence for this? • Crossing yellow never produces exclusively yellow offspring: a ratio of two yellow to one grey always results. This means all viable yellow mice are heterozygous and a living homozygous yellow mouse is an impossibility. • Examination of the uteri of yellow female mice which have mated with yellow males usually reveals one of more dead embryos.

12. Is this allele dominant or recessive? • It depends how we look at it. • The allele controlling fur colour is dominant, but as a lethal allele it is recessive, exerting its effects only when in the homozygous state.

13. Lethal alleles in fruit flies, Drosophila melanogaster

14. In Drosophila fruit flies, a mutated allele which is dominant (C ) produces curly wings rather than normal wings (c ) • Fruit flies that are homozygous for curly wings do not survive. • The expected 3 curly :1 normal ratio of flies does not occur • However 2 curly: 1 normal does occur. • The homozygous dominant flies do not survive. • Draw a punnet square to show this.

15. Manx Cats

16. Cats with tails have the genotype tt – but the tail-producing gene has a dominant mutation. • In heterozygous cats, Ttthe allele results in the tail-less Manx phenotype. • However, when the homozygous dominant occurs, the alleles cause problems with spinal development, which are lethal. • Draw a punnet square to show how the expected 3:1 ratio actually produces a 2:1 ratio.

17. Snapdragon plants and chlorophyll • Snapdragon plants can carry a dominant allele (G) that causes plants to be unable to make chlorophyll. • Chlorophyll is essential for photosynthesis and therefore survival, so plants that inherit two copies of the dominant allele die soon after germination. • The heterozygous offspring have golden leaves Draw a punnet square to show this.

18. Creeper allele in chickens

19. The Creeper allele • The dominant creeper allele (C ) in chickens causes the legs to be shortened and stunted. • If two creeper chickens are mated you would expect to ¾ offspring to be creeper and ¼ to be normal. • Instead the ratio obtained is 2/3 creeper and 1/3 normal. • Homozygous creeper chickens die before hatching. • Draw a punnet square to show this.