Mendel’s Second Law (Law of independent assortment)

1. Mendel’s Second Law (Law of independent assortment) • It states that gens coding for different characteristics separate independently of one another when gametes are formed owing to independent separation of homologous pairs of chromosomes during meiosis • This principle states that alleles at different loci separate independently of one other

2. Mendel's 2nd Law – the Law of Independent Assortment This law stats that: • When two pairs of contrasting traits are brought in the same cross, they together in offspring of F1 generation but assort independently at meiosis (in F2 generation)

4. This means that • Homologous chromosomes and alleles segregate at meiosis it one to one ratio. • Non-homologous chromosomes along with their genes separate and recombine again in new combinations at meiosis independently.

5. Mendel's 2nd Law – the Law of Independent Assortment Two types of crosses: • Dihybrid Self crosses • Dihybrid Test crosses

6. Dihybrid Self crosses • Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb) • Parental Cross: Yellow, Round Seed x Green, Wrinkled • F1 Generation: All yellow, round • F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled • Seed Color: Yellow = G; Green = g • Seed Shape: Round = W; Wrinkled = w

7. Dihybrid Self crosses • Parental Phenotypes: Yellow Round X Green Wrinkled • Parental genotypes: YYRR X yyrr • Gametes: YR yr • F1: YyRr ( All Yellow Round) • Self cross(F2): F1 X F1 • Parental Phenotypes: Yellow Round X Yellow Round Parental genotypes: YyRrX YyRr

8. Gametes:

9. F2 ratios of Independent assortment are calculated by two methods: • By multiplying segregation ratios (9: 3: 3: 1) • By checker board (Punnet square) (9: 3: 3: 1)

10. By multiplying segregation ratios

11. By checker board (Punnet square) • Parents: F1 X F1 • Parental Phenotypes: Yellow Round X Yellow Round • Parental genotypes: YyRrX YyRr • Gametes: YR , Yr, yR, yr X YR , Yr, yR, yr

12. Dihybrid Crosses Test Cross: • F1 (Dihybrid Crosses) offspring is crossed with recessive parent: • Parental Phenotypes: Yellow Round X GreenWrinkled • Parental genotypes: YyRrXyyrr • Gametes: YR , Yr, yR, yr all yr

13. This test cross ratio tell that non-homologous chromosomes assort independently.

14. Backcross • Mendel crossed two varieties of peas that differed in height, He established that tall (T) was dominant over short (t) • He tested his theory concerning the inheritance of dominant traits by crossing an F1 tall plant that was heterozygous (Tt) with the short homozygous parental variety (tt) • This type of cross between an F1 genotype and either of the parental genotype is called backcross

16. Punnet Square • It is constructed by drawing a grid putting the gametes produced by one parent along the upper edge and the gametes produced by the other parent down the left side

17. Punnett Square (Checkered board) Why it is used? • Help to predict the results of experimental crosses. • To determine the kind of gametes each parent produces. For this purpose, • One of the two axes of a square is designated for each parent, and the different kinds of gametes, each parent produces are listed along the appropriate axis. • Combining the gametes in the interior of the square shows the results of random fertilization. Ratios for test cross: 1:1:1:1 Ratios of self cross : 9:3:3:1 • Hence proved non-homologous chromosome assort independently.

18. Sex DeterminationSex linked inheritance Lecture 3 Dr. AttyaBhatti

19. Sex Determination • Sex refers to sexual phenotype • Two sexual phenotypes: male and female • Difference between males and females is gamete size: • males produce small gametes; • females produce relatively large gametes • Mechanism by which sex is established is termed sex determination

20. Sex Determination • Cells of female humans have two X chromosomes • Cells of males have one X chromosome and one Y chromosome • Ways in which sex differences arise: • Hermaphroditism ( that has only bisexual reproductive units) • Monoecious (an individual that has both male and female reproductive units) • Dioecious (refers to a plant population having separate male and female plants.)

21. Chromosomal Sex-Determining Systems • Sex chromosomes: differ between males and females • Autosomes: nonsex chromosomes which are the same for males and females • XX-XO sex determination • XX-XY sex determination • ZZ-ZW sex determination

22. XX-XO sex determination • Sex determination in the grasshoppers studied by McClung • In this system • Females have two X chromosomes (XX) • Males possess a single X chromosome (XO) • No O chromosome (O signifies the absence of a sex chromosome)

23. XX-XO sex determination • In females: the two X chromosomes pair and then separate with one X chromosome entering each haploid egg • In males: the single X chromosome segregates in meiosis to half the sperm cells, the other half receive no sex chromosome

24. XX-XY Sex Determination • Cells of males and females have the same number of chromosomes • Cells of females have two X chromosomes (XX) • Cells of males have a single X chromosome and a smaller sex chromosome called the Y chromosome (XY) • Male is the heterogametic sex • Female is the homogametic sex

26. XX-XY Sex Determination • X and Y chromosomes are not generally homologous do pair and segregate into different cells in meiosis • Pseudoautosomal Regions • In humans there are pseudoautosomal regions at both tips of the X and Y chromosomes

27. The X and Y chromosomes in humans differ in size and genetic content

28. ZZ-ZW Sex Determination • Female is heterogametic • Male is homogametic • Sex chromosomes are labeled Z and W • Females in this system are ZW • Males are ZZ • ZZ-ZW system is found in: • Birds,moths, some amphibians, and some fishes

29. Haplodiploidy • Insects possess haplodiploid sex determination • Males develop from unfertilized eggs and are haploid • Females develop from fertilized eggs and are diploid

30. In insects with haplodiploidy, males develop from unfertilized eggs and are haploid; females develop from fertilized eggs and are diploid

31. Sex Determination in Drosophila • Fruit fly Drosophila melanogaster has eight chromosomes • Three pairs of autosomes • One pair of sex chromosomes • Females have two X chromosomes • Males have an X chromosome and a Y chromosome

32. Life cycle of Drosophila melanogaster, the common fruit fly.

33. The sexual phenotype of a fruit fly is determined by the ratio of the number of X chromosomes to the number of haploid sets of autosomal chromosomes (the X:A ratio) The chromosomes of Drosophila melanogaster

34. From: Sex chromosomes and sex-linked in Sex Determination in Humans • XX-XY sex determination • Presence of a gene on the Y chromosome determines maleness • Which arise when the sex chromosomes do not segregate properly in meiosis or mitosis? • Turner syndrome • Klinefelter syndrome • Poly-X females

35. Chromosomal Determination of Sex in Drosophila and Humans

36. Persons with Turner syndrome have a single X chromosome in their cells Persons with Klinefelter syndrome have a Y chromosome and two or more X Chromosomes in their cells

37. The Role of Sex Chromosomes in Humans • X chromosome contains genetic information • Male-determining gene is located on the Y chromosome • Absence of the Y chromosome results in a female phenotype • Genes affecting fertility are located on the X and Y chromosomes • Additional copies of the X chromosome may upset normal development

38. The Male-Determining Gene in Humans • Sex-determining region Y (SRY) gene • Found in XX males • Missing from all XY females • SRY gene on the Y chromosome causes a human embryo to develop as a male • Absence of this gene a human embryo develops as a female

39. The SRY gene is on the Y chromosome and causes the development of male characteristics