- 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)
- 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.
- lethal genes - genes that leads to the death.
- - 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
If we cross two heterozygotes, expect a 3:1 ratio in progeny But, in yellow x yellow cross, get: 2 yellow : 1 nonyellow Explanation:
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)
- Example of dominant lethal allele
- 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.
Inheritance of Huntington’s allele
- Genes that are on the same
- Such genes do not obey Mendel’s laws because they do not undergo
- They are inherited together unless
- separated by crossing over during
Linked genes in Drosophila
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.
- 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.
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.
- 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.
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
Transmission of sex-linked genes