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  • 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:
  • 5.  
  • 6.  
  • 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.
  • 14.  
  • 15.  
  • 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.
  • 18.  
  • 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
  • 20.  
  • 21. Transmission of sex-linked genes