Dihybrid Crosses and Gene Linkage

1. Dihybrid Crosses and Gene Linkage

2. Assessment Statements (objectives) 10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.2 Distinguish between autosomes and sex chromosomes. 10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles. 10.2.4 Define linkage group. 10.2.5 Explain an example of a cross between two linked genes. 10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

3. Dihybrid Crosses • Gregor Mendel experimented with pea plants • In one cross he examined 2 traits: • Seed shape • Round vs wrinkled • The allele for round is dominate • Seed color • Green vs yellow • The allele for yellow is dominate

4. Dihybrid Crosses • Gregor Mendel crossed 2 true-breeding plants with each other • True-breeding means homozygous for the traits being studied • One parent plant was homozygous for both dominate traits (round and yellow) • One parent plant was homozygous for both recessive traits (wrinkled and green)

5. Dihybrid Crosses • R = allele for round peas • r= allele for wrinkled peas • Y = allele for yellow peas • y = allele for green peas

6. Dihybrid Crosses Parent phenotypes: Round yellowand Green Wrinkled Parent genotypes:RRYY rryy Parent gametes: RY ry F1 genotypes: RrYy F1 phenotypes: round yellow

7. Dihybrid Crosses

8. Dihybrid Crosses • The F1 generation were all round (RR) and yellow (YY) • When these peas were planted and self pollinated, Mendel expected some of the recessive traits to show up again, and they did • Of 556 peas: • 315 round and yellow (56.6%) • 101 wrinkled and yellow (18.2%) • 108 round and green (19.4%) • 32 wrinkled and green (5.8%) • When converted into ratios, numbers are 9:3:3:1 • This can be calculated using a 4 x 4 Punnett square

9. Dihybrid Crosses

10. Dihybrid Crosses • When making a solving a Punnett square always include: • Genotype of parents • Key of what the letters mean • Phenotypes of the offspring • Alleles found in the gametes

11. The phenotypes and general genotypes from this cross can be represented in the following manner: The results of this experiment led Mendel to formulate his second law. Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair

12. Autosomes and sex chromosomes • Any chromosome which is not a sex chromosome (X or Y) is and autosomal chromosome • Humans have 22 pairs of autosomes and one pair of sex chromosomes • If a trait or gene is called autosomal its locus in on one of the 22 autosomes • A trait or gene which is said to be sex-linked must have its locus on a sex chromosome

13. Autosomes and sex chromosomes • Where a gene is located determines whether or not the trait it controls is more common in males or females • When a trait is more common in one sex that the other, chances are that the trait is sex-linked • If there is no pattern to the frequency of a trait between males or females, it is more likely an autosomal trait

14. Autosomes and sex chromosomes

15. Exchange of alleles by crossing over • Two non-sister chromatids can swap segments of their DNA • The maternal chromosome can end up with a segment of genetic material from a paternal chromosome and vice versa • Thus a chromosome originally carrying a recessive allele could end up with a dominant allele that traded during crossing over.

16. Exchange of alleles by crossing over • Consider a bivalent in which the maternal chromosome has allele B for an autosomal trait and the paternal chromosome has the recessive allele b. • When crossing over is complete, the segments containing the locus of the gene have been swapped and the alleles have switched places. • Now the paternal chromosome contains B and the maternal contains b. • The sister chromatids are no longer identical

17. Exchange of alleles by crossing over

18. Exchange of alleles by crossing over • During any single cross over event, hundreds or thousands of genes can be traded between non-sister chromatids • A single bivalent can have several chiasmata producing cross over in more than one chromatid • This is another source of variation in sperm and egg cells

19. Linkage Group • Any two genes found on the same chromosome are said to be linked to each other • Linked genes are usually passed on to the next generation together • A group of genes inherited together because they are found on the same chromosome are considered to be members of a linkage group • Mendel is lucky he didn’t chose traits that were not linked

20. Linkage Group

21. Linked Genes Video Clip • http://bcs.whfreeman.com/thelifewire/content/chp10/1002002.html

22. Linked Genes • In fruit fly, Drosophila, the gene for body color (grey or black) is in the same linkage group as the gene for wing length (long or short) • G = allele for grey body • g = allele for black body • L = allele for long wings • l = allele for short wings

23. Linked Genes • The genotype of true-breeding (homozygous) parents are: • GGLL = genotype for grey body and long wings • ggll = genotype for black body and short wings • In order to show linkage, the following notation is used: G L G L

24. Linked Genes • The 2 horizontal bars symbolize homologous chromosomes and show that the locus of G is on the same chromosome as L. • One G is on the maternal homologue and the other is on the paternal homologue G L G L

25. Linked Genes • Likewise, ggll is shown: • The pairs of the alleles are read vertically: the above symbol’s genotype is ggll g l g l

26. Offspring of a dihybrid cross • A cross between a homozygous dominate true-breeding fruit fly (GGLL) and a homozygous recessive true-breeding fruit fly (ggll) would result in heterozygous for both traits (GgLl) • All the flies would be grey with long wingsand carriers for the recessive traits • The offspring are different from either parent • A new shuffling of the alleles has created a new combination which does not match either of the parent’s genotypes • The term that describes the new chromosome is recombinant

27. Offspring of dihybrid cross • Linked genes in heterozygote: • The pairs of the alleles are read vertically: the above symbol’s genotype is ggll G L g l

28. Offspring of dihybrid cross • The recombinants form through crossing over • Without crossing over, the allele G would always be inherited with L because they are linked • With crossing over, G sometimes gets inherited with l, and g sometimes with L • Linked genes don’t follow Mendel's law of independent assortment • Linked genes tend to stay together because they are physically close together