Chromosomal Basis of Inheritance

1. Chromosomal Basis of Inheritance Section 10.6 and Chapter 11

2. Chromosomes are the Physical Basis of MendelianInheritance • chromosomal behavior accounts for Mendel's laws and ratios • movement of chrom. during meiosis  Mendel’s laws and ratios • chromosome theory of inheritance • human chromosomes • 22 pairs of autosomalchromosomes • normal body chromosomes • 1 pair of sex chromosomes (X and Y) • males: XY; females: XX • all chromosomes contain thousands of genes • Linked Genes • genes located on the samechromosome • genes located specifically on sex chromosomes = sex-linked • linked genes tend to be inherited together – why? • they do not assort independently • crosses involving them deviate from Mendel's laws and ratios • parental pheno. are disproportionately represented in offspring • even with linked genes, some offspring have traits different from parents • crossing over

3. Linked genes and their effects on inheritance

4. A closer look at crossing over

5. Genetic Recombination • recombinant offspring • offspring with traits different from their parents • independent assortment • Mendel's law • recombination of unlinkedgenes • genes on different chromosomes • takes place during meiosis I (metaphase) • yields typical Mendelianratios • crossing over (crossover) • recombination of linked genes • takes place during meiosis I (prophase) • genes located farther apart are more likely to crossover • gene mapping • used to determine the order and position of genes on chrom. • mapping techniques make use of manychrom. features • several different kinds of maps and many uses of gene mapping

6. A genetic map of a fruit fly chromosome

7. Sex Chromosomes and Sex-Linked Genes • sex of any organism has a chromosomal basis • varies by type of organism involved • sex is an inherited trait determined by certain chrom. • X-Y system  females: XX; males: XY • X-0 system  females: XX; males: X • Z-W system  females: ZW; males: ZZ • haploid-diploid system  females: 2n; males: n • sex-linked genes • have unique patterns of inheritance • sex chrom. carry most genes related to sex • also carry genes unrelated to sex • almost all sex-linked genes are carried only on the X-chrom. • nocorresponding gene on the Y • thus, sex-linked traits are said to be “X-linked” • males have only one copy of these X-linked genes – why? • females have 2 copies of these X-linked genes – why? • females can be heterozygous for any X-linked trait, whereas males cannot be

8. several X-linked genes are the cause of sex-linked disorders • most of these disorders are recessive • found much more frequently in males than in females – why? • females must inherit 2 copies of the recessive gene, while males need only inherit one – why? • sex-linked genetics problems (see handout) • hemophilia: X-linked recessive trait, causes blood to clot improperly • H = normal allele; h = hemophilia allele • genotypes: • XHXHand XHXh= normal female • XHXh= carrier female • XhXh= hemophiliac female • XHY = normal male • XhY= hemophiliac male

9. Sex-Linked Genes • Problem: In humans, hemophilia is an X-linked recessive trait. A hemophiliac man has a daughter with the normal phenotype. She meets a man who is also normal for the trait. What are the genotypes of everyone involved? What is the probability that the couple will have a hemophiliac daughter? A hemophiliac son? If the couple has 3 sons, what chance is there that all of them will have hemophilia? • Answer: • Part 1: Determine the genotypes of everyone involved. • hemophiliac man = XhY (by definition) • normal man the daughter meets = XHY (by definition) • normal daughter = she must be XHXh, regardless if her mother was XHXH or XHXh • Check this with Punnett Squares:

10. Mother XHXH • Mother XHXh XHXH Xh Y XHXh Xh Y

11. XHXh • Part 2: Determine the possibilities for the couple's offspring. • The F1 cross is XHXhx XHY • Part 3: State these possibilities as probabilities. • normal daughter (XHXH or XHXh) = 50% = 1/2 = (1 in 2) • carrier daughter (XHXh) = 25% = 1/4 = (1 in 4) • hemophiliac daughter (XhXh) = 0% • normal son (XHY) = 25% = 1/4 = (1 in 4) • hemophiliac son (XhY) = 25% = 1/4 = (1 in 4) • chance of 3 sons being hemophiliacs (use Rule of Multiplication): • 1/4 x 1/4 x 1/4 = 1/64 = 1.6 % chance XH Y

12. Fig. 11.16 X-linked inheritance

13. Errors in Chromosomal Inheritance • genetic disorders can be caused by: • recessive alleles on any chromosome, esp., X-linked recessives • physical/chemical disturbances that damage chrom. or alter inheritance • errors in meiosis that alter inheritance • nondisjunction – an error during meiosis • can occur in two ways: • homologous chromosomes fail to separate (meiosis I) • sister chromatids fail to separate (meiosis II) • one gamete receives two of the same chrom., the other receives no copy • abnormal gamete unites with normal one at fertilization • aneuploidy

14. Fig. 10.10 Nondisjunction

15. trisomy • aneuploid cell has a chromosome in triplicate (2n + 1) • trisomy 21 = Down's Syndrome • trisomy 18 = Edward’s Syndrome • Poly-X (XXX) • Klinefelter's Syndrome (XXY) • Jacob’s Syndrome (XYY) • monosomy • aneuploidcell has only 1 copy of a certain chrom. (2n - 1) • almost all cases are lethal • monosomyX (X0) = Turner’s Syndrome • tetrasomy(2n + 2), pentasomy (2n + 3), etc. • rare and usually involve only sex chrom. Fig. 10.11 A child with Down’s Syndrome. Note the karyotype showing an extra chromosome #21

16. Fig. 10.12 Turner’s Syndrome (XO) and Klinefelter’s Syndrome (XXY)

17. polyploidy • organism possesses more than two complete sets of chrom. • triploidy(3n) and tetraploidy (4n) • common in plant kingdom; very rare in animals • can result from complete nondisjunction during meiosis • polyploidsare more nearly normal than aneuploids – why? • mosaicism • chrom. abnormalities that do not show up in every cell • only present in some cells and tissues • an ind. has two populations of cells with different genotypes • both came from a single fertilized egg • usually results from mutationsin mitosis, early in embryonic devel. • symptoms less severe than if all cells are affected

18. Heterochromia Blashko Lines Examples of mosaicism

19. structural alterations of chromosomes • alterations in the physical/chemical structure of chrom. • most have harmful effects; but some beneficial • deletions • duplications • often have beneficial effects  major evol. mechanism • inversions • translocations

20. Fig. 10.13 Types of chromosomal mutations

21. Fig. 10.14. The results of a deletion. When chromosome #7 loses an end piece, the result is Williams Syndrome. These children, although unrelated, have the same appearance, health, and behavioral problems

22. Another result of a deletion. When a group of genes are accidentally deleted from chromosome #5, the result it Cri du Chat syndrome.

23. Fig. 10.15 The results of a translocation. When chromosomes #2 and #20 exchange segments, the result is Alagille Syndrome. Individuals have distinctive facial features because the translocation disrupts an allele on chromosome #20.