2. Gene Linkage Inheritance Patterns for ---Two Genes on the Same Chromosome ---Genes on the Sex Chromosomes

3. Sex Determination Sex Chromosomes : homologous chromosomes that differ in size and genetic composition between males and females Sex Chromosomes : homologous chromosomes that differ in size and genetic composition between males and females HumanChromosomeXY SizeLargerSmaller GeneticCompositionMultiplegenesunrelated to gender Very few genes SRY region determines maleness

4. Sex Determination Autosome: any chromosome that is not a sex chromosome Autosome: any chromosome that is not a sex chromosome Humans have 22 autosome pairs and 1 pair of sex chromosomes

5. Sex Determination Female Male XX x XY XX x XY XY X X XXXX XY XY Phenotypic Ratio of Offspring ½ Female + ½ Male ½ Female + ½ Male

6. Sex Linked Genes Hemophilic Male Non-hemophilic Female Female (father is hemophilic) (father is hemophilic) XhXhYYXhXhYYY X H X h XHXhXHXhXHXhXHXh XhXhXhXhXhXhXhXh XHYXHYXHYXHY XhYXhYXhYXhY Phenotypic Ratio of Offspring ¼ hemophilic males + ¼ non-hemophilic males ¼ hemophilic females + ¼ non-hemophilic females X h Y x X H X h Males are hemizygous for traits on the X chromosome X chromosome Females can be homozygous or heterozygous for traits on the X chromosome X chromosomeDad X h Y XhXhXhXh

7. Patterns of Inheritance Autosomal DominantAutosomal Dominant Autosomal RecessiveAutosomal Recessive Sex-Linked Dominant (X-linked)Sex-Linked Dominant (X-linked) Sex-Linked Recessive (X-linked)Sex-Linked Recessive (X-linked)

8. Pedigree for An Autosomal Dominant Trait

9. Pedigree for Albinism: An Autosomal Recessive Trait

10. Pedigree for Hemophilia, An X-linked Recessive Trait

11. Pedigree for Colorblindness, An X-linked Recessive Trait

12. Patterns of Inheritance Autosomal DominantAutosomal Dominant Autosomal RecessiveAutosomal Recessive Sex-Linked Dominant (X-linked)Sex-Linked Dominant (X-linked) Sex-Linked Recessive (X-linked)Sex-Linked Recessive (X-linked) Mitochondrial InheritanceMitochondrial Inheritance

13. Linked Genes Mendel’s experiments Linked Genes Gene location Genes on separate chromosomes or far apart on the same chromosome Genes on the same chromosome Gamete types Equal numbers of all possible allele combinations More parental combinations than recombinant combinations

14. How Do We Study Gene Linkage? Example from the Fruit Fly Example from the Fruit Fly Red eyes, x Pink eyes Red eyes, x Pink eyes Beige body Ebony body Beige body Ebony body RRBBrrbb RRBBrrbb F1: Red eyes, Beige body RrBb F1: Red eyes, Beige body RrBb

15. Testcross: cross to individual of known genotype F1: Red eyes X Pink eyes F1: Red eyes X Pink eyes Beige body Ebony body Beige body Ebony body RrBbrrbb RrBbrrbb How Do We Study Gene Linkage?

16. F2 phenotype Number of Offspring Expected for Unlinked Genes Red eyes Beige body 398250 Pink eyes Ebony body 382250 Red eyes Ebony body 108250 Pink eyes Beige body 112250

17. How Do We Study Gene Linkage? If genes are linked: Red eyes, x Pink eyes Red eyes, x Pink eyes Beige body Ebony body Beige body Ebony body r b R B F1: Red eyes, Beige body F1: Red eyes, Beige body R B r b

18. F1: Red eyes, Beige body F1: Red eyes, Beige body How Do We Study Gene Linkage? R B r b X R B r b r B R b Four types of gametes are produced Parental Recombinant Four types of gametes are produced Parental Recombinant Alleles are in Cis Configuration

19. How Do We Study Gene Linkage? F1:Red eyes X Pink eyes F1:Red eyes X Pink eyes Beige body Ebony body Beige body Ebony body R B r b R b r B R B r b R B r b R b r b r B r b

20. How Do We Study Gene Linkage? F2 phenotype Number of Offspring Chromosome arrangement Red eyes Beige body 398RB//rbParental Pink eyes Ebony body 382rb//rbParental Red eyes Ebony body 108Rb//rbRecombinant Pink eyes Beige body 112rB//rbRecombinant

21. Genetic Map Units 1% recombination = 1 map unit 108 + 112 x 100 = 22% 108 + 112 x 100 = 22% 1000 1000 These genes are located 22 map units apart on the same chromosome. These genes are located 22 map units apart on the same chromosome. Map is drawn as: R B R B I 22 map units I I 22 map units I

22. Knowing the gene distance allows us to predict the expected percentages of each type of gamete. Knowing the gene distance allows us to predict the expected percentages of each type of gamete. How Do We Study Gene Linkage? Parental Recombinant Parental Recombinant X r B R b r B r B R b 39%39% 78% R B r b 11%11% 22% Alleles are in Trans Configuration

23. Gene Linkage Summary A.Linked genes tend to be inherited together in their original, parental arrangement. B. Alleles of linked genes can be separated by crossing over (recombination) during meiosis. C.Distance between genes is calculated as 1% recombination = 1 map unit. D.Gene distances can be calculated for genes that are less than 50 map units apart. At 50 map units and beyond, we cannot distinguish between genes far apart on the same chromosome and genes on separate chromosomes.