Inheritance Patterns for Linked Genes

1. Inheritance Patterns for LinkedGenes

2. Section Summary • Genes are located at specific chromosome locations called loci (singular, locus). • Genes located in close proximity to each other on a chromosome tend to be genetically linked. (the genes are inherited together) • Traits linked to a particular sex chromosome exhibit unique inheritance patterns.

3. Gene Loci • The alleles of a gene reside at the same location, or locus, on homologous chromosomes.

4. Gene Loci • Homologous chromosomes may bear either the same alleles or different ones at a particular locus, making an organism either homozygous or heterozygous for each gene.

5. Independent Assortment

6. Genetic Linkage • If genes are located on separate chromosomes, they assort independently of each other during meiosis.

7. Genetic Linkage • The alleles for genes located on the same chromosomeare less likely to assort independently, and instead stay together during meiosis. • The only way such alleles can assort independently is if crossing over during meiosis separates them.

10. Genetic Linkage • The tendency for alleles for different genes on the same chromosome to be inherited together is called genetic linkage. • Genes with loci that are close together are known as linked genes.

11. Genetic Linkage • The closer two genes are on a chromosome, the greater the degree of genetic linkage • The farther apart the genes are, the more likely it is that crossing over will separate them.

12. Gene Maps • Recombination frequency,(the frequency with which certain genes turn up together) can be used to map the distance between gene loci on a chromosome. • These values allow geneticists to create a gene map, a diagram that shows the relative locations and distances of genes on a chromosome.

14. Gene Maps • This is a gene map of one of a rat’s chromosomes

15. Inheritance Patterns of Sex-Linked Traits • Any gene that is located on a sex chromosome is called a sex-linked gene. Scanning electron micrograph of an X and Y chromosome Image courtesy of Indigo Instruments, Canada

16. Inheritance Patterns of Sex-Linked Traits • In humans, most sex-linked genes are found on the X chromosome, which is considerably larger in size than the Y chromosome. • Genes on the X chromosome are said to be X-linked.

18. Inheritance Patterns of Sex-Linked Traits • The gene involved in the white-eye inheritance pattern in fruit flies is located only on the X chromosome. • There is no corresponding gene for eye-colour on the Y chromosome.

19. Inheritance Patterns of Sex-Linked Traits • Females have two X chromosomes (they are XX), therefore females carry two copies of every X-linked gene. • Males have only one X chromosome (they are XY) and so carry only one copy of the X-linked gene.

20. F1 Generation F2 Generation

21. Inheritance Patterns of Sex-Linked Traits • Since the white-eye trait is recessive, a female will have white eyes only if both X chromosomes carry the white-eye allele (XrXr). • If a male inherits a single copy of the white-eye allele on his X chromosome, as there is no other copy of the gene, he will have white eyes (XrY).

22. Sex-Linked Disorders in Humans

23. Sex-Linked Disorders in Humans Red-Green Colour blindness Hemophilia

24. Sex-Linked Inheritance • A "carrier" is a female who is heterozygous for the recessive, sex-linked trait.

25. Sex-Linked Inheritance • A female "carrier" who is heterozygous for the recessive, sex-linked trait red color blindness, marries a normal male. What are the possible phenotypes of the offspring?

26. Sex-Linked Inheritance • Let XR represent the gene for normal colour vision • Let Xr represent the gene for red-green colour blindness

27. Example • A female carrier for hemophilia has children with an unaffected male. What are the possible phenotypes of their children?

29. Hemophilia

30. Homework • Page 151 # 1 - 11