The Chromosomal Basis of Inheritance

1. The Chromosomal Basis of Inheritance Chapter 15

2. Agenda • Warm up: Hemophilia case study • Discuss genetics problems in group and as class • Review blue people case study • Lecture: chromosome theory of inheritance, chromosomal basis of sex, and inheritance of sex-linked alleles • Exit Slip Learning Outcome: I can explain the chromosomal theory of inheritance. I can solve problems related to the transmission of sex-linked recessive traits. I can

3. The Chromosomal Theory of Inheritance • Mendelian genes have specific loci (positions) along chromosomes. • It is the chromsomes that undergo segregation and independent assortment.

4. Fig. 15-2 P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) y Y r  R r R Y y Meiosis Fertilization r y Y R Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r r R R Metaphase I y Y y Y 1 1 r r R R Anaphase I Y y Y y Metaphase II r R R r 2 2 y Y y Y y Y Y y Y y y Y Gametes r R r r R r R R 1/4 1/4 1/4 1/4 yr yR YR Yr F2 Generation An F1  F1 cross-fertilization 3 3 : 1 : 3 9 : 3

5. Thomas Hunt Morgan • Drosophila melanogaster • Wild Type- the phenotype for a character most commonly observed in natural populations. • Mutant Phenotypes- traits that alternatives to the wild type. • A student at UK

6. Fig. 15-4b EXPERIMENT P  Generation F1 All offspring had red eyes Generation RESULTS F2 Generation

7. The Chromosomal Basis of Sex Determination • SRY- Sex determining Region of Y • -The presence of this gene on the Y chromosome • codes for the development of testes. In the absence of • this gene, the gonads develop into ovaries

8. Sex Linked Genes • A gene located on either sex chromosome is called • a sex-linked gene • In humans, sex-linked usually refers to a gene on • the larger X chromosome • For a recessive sex-linked trait to be • expressed • -A female needs two copies of the allele • -A male needs only one copy of the allele • Sex-linked recessive disorders are much • more common in males than in females

9. Fig. 15-7 N= normal color vision XnY XnY XNXN XNXn  XNY XNXn  

10. Fig. 15-7 XnY XnY XNXN XNXn  XNY XNXn   Sperm Sperm Sperm Xn Y XN Xn Y Y Eggs Eggs XNXN Eggs XNXn XNY XN XNY XN XNXn XNY XN XnXn XnY XNXn XNY XnXN XnY Xn Xn XN (a) (b) (c)

11. Morgan’s Experiments • Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses.

12. What type of ratios would you expect to see in the testcross offspring if the genes were located on different chromosomes? What if they were located on the same chromosome and parental alleles are always inherited together?

13. Morgan’s Experiments • Linked Genes- genes located on the same chromosome that tend to be inherited together in genetic crosses. • Genetic Recombination- the production of offspring with combinations of traits that differ from those found in either parent.

14. Parental Types- offspring that inherit a phenotype that matches one of the parental phenotypes. Gametes from yellow-round heterozygous parent (YyRr) Yr YR yr yR Gametes from green- wrinkled homozygous recessive parent (yyrr) yr yyRr YyRr yyrr Yyrr Parental- type offspring Recombinant offspring Recombinant Types or Recombinants- offspring that have new combinations of phenotypes.

15. Recombination • Crossing Over- accounts for the recombination of linked genes. • Genetic Map- an ordered list of the genetic loci along a particular chromosome. • Linkage Map- a genetic map based on recombination frequencies. Displays order but not precise location. • Distances are expressed in map units- equivalent to 1% recombination frequency. (Centimorgans)

16. Exit Slip • In humans, pseudohypertrophic muscular dystrophy is a condition in which the muscles gradually waste away, ending in death in the early teens. This type of muscular dystrophy is dependent upon a sex-linked recessive gene that occurs only in boys and has never been reported in girls. Hypothesize why it does not appear in girls? • Why does the sex-linked gene for pseudohypertrophic muscular dystrophy not become eliminated from the human race since all boys showing the trait die before reaching maturity?

17. Alteration of Chromosome Number • Nondisjunction- pairs of homologous chromosomes do not separate normally during meiosis • As a result, one gamete receives two of the same type of chromosome and the other gamete doesn’t receive a copy. • Aneuploidy- a chromosomal aberration in which one or more chromosomes are present in extra copies or deficient in number • Polyploidy- a chromosomal alteration in which the organism possesses more than two complete chromosome sets.

18. Fig. 15-13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n – 1 n + 1 n – 1 n n n + 1 n + 1 n – 1 Number of chromosomes (b) Nondisjunction of sister chromatids in meiosis II (a) Nondisjunction of homologous chromosomes in meiosis I

19. Alteration of Chromosome Structure • Deletion- removes a chromosomal segment • Duplication- repeats a segment • Inversion- reverses a segment within a chromosome • Translocation- moves a segment from one chromosome to another

20. Exceptions to the Standard Chromosome Theory • Genomic Imprinting- a variation in phenotype depending on whether an allele is inherited from the male or female parent. • Extranuclear Genes- genes outside of the nucleus. Some are located in organelles in the cytoplasm.