This document discusses different types of genes and inheritance patterns: 1) Lethal genes can be dominant or recessive, with recessive lethal alleles following a 2:1 genotypic to phenotypic ratio as seen in mouse coat color. 2) Linked genes on the same chromosome are inherited together and do not follow Mendel's laws of independent assortment unless separated by crossing over. This is demonstrated using examples in fruit flies. 3) Sex is determined by X and Y chromosomes in humans, with females as XX and males as XY. Sex-linked genes on the X chromosome are inherited differently between males and females.

1. LECTURE 5 2.2.5 Lethal Genes 2.2.6 Linked Genes

2. OBJECTIVES At the end of the lesson, students should be able to : Explain dominant and recessive lethal allele. Calculate genotypic and phenotypic ratios for recessive lethal alleles (2:1) Explain linked genes. Show the effect of linked genes on the dihybrid test cross (3:1) in the absence of crossing over. Show the effect of linked genes with crossing over on the dihybrid test cross ratio. Explain sex-linked genes.

3. Lethal Genes lethal genes - genes that leads to the death. Types of lethal genes. - Dominant lethal allele - recessive lethal allele Example for recessive lethal allele E.g: gene for coat color of rodents (mice). Wild mice have grey-colored fur (agouti) while mutants, yellow. A self cross between mice with yellow fur produces offspring in the ratio of 2 yellow to 1 agouti. These results suggest that the allele for yellow ( Y ) is dominant to the allele for agouti ( y ). So, although the Y allele is dominant for fur color , it is recessive for the lethal characteristic . Genotypic and phenotypic ratio is 2:1

4. If we cross two heterozygotes, expect a 3:1 ratio in progeny But, in yellow x yellow cross, get: 2 yellow : 1 nonyellow Explanation:

7. Coat color of rodents P : Yellow X Yellow Yy Yy G : Y y Y y F1: YY Yy yY yy Yellow Yellow Yellow Grey (Die)

8. Example of dominant lethal allele Hungtinton’s disease A degenerative disease of the nervous system Has no obvious phenotypic effect until certain age. If Hh x hh , with H is dominant allele for Hungtinton’s disease, 50% of child born inheriting the allele & the disorder.

9. Inheritance of Huntington’s allele

10. Linked Genes Genes that are on the same chromosome are linked. Such genes do not obey Mendel’s laws because they do not undergo independent assortment. They are inherited together unless separated by crossing over during prophase 1 of meiosis.

11. Linked genes in Drosophila

12. SEX DETERMINATION IN HUMANS Human sex is determined by a pair of sex chromosomes called X and Y . Because these chromosomes do not look alike, they are called heterosomes . All other chromosomes are called autosomes . Every human cell contains 23 pairs of chromosomes.

13. Females have two large X chromosomes ( XX ), males have one X chromosome and one Y chromosome ( XY ). During meiosis, the sex chromosomes pair up and segregate into the daughter cells. Males are heterogametic sex because they produce different sperm : approximately 50% contain an X chromosome and 50% have a Y chromosome. Females produce homogametic sex because all of their egg contain an X chromosome.

16. Sex Linked Genes Human have 22 pairs of autosomes and a pair of sex chromosome. Genes carried on the sex chromosomes are said to be sex-linked . Human females have two X chromosomes , meaning they have two sex-linked alleles.

17. In males, the Y chromosome is smaller and cannot mirror all the genes found on the X chromosome, so males have only one sex-linked allele. This is why males suffer from the effects of X-linked genetic diseases more often than females. There are no known Y-linked traits, probably because the Y chromosome carries so few genes.

19. X c X c Color blind X X C Y Normal X C X c X C X c X c Y X c Y Parent gamete progeny 100% carrier 100% color blind X c X c X C Y

21. Transmission of sex-linked genes