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Chromosomal Basis of Inheritance

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1 Chromosomal Basis of Inheritance
Section 10.6 and Chapter 11

2 Chromosomes are the Physical Basis of Mendelian Inheritance
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 autosomal chromosomes 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 same chromosome 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 unlinked genes genes on different chromosomes takes place during meiosis I (metaphase) yields typical Mendelian ratios 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 many chrom. 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. no corresponding 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: XHXH and 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 XH XH XHXh XHY Xh Y XH Xh XHXh XhXh XHY XhY Xh

11 Part 2: Determine the possibilities for the couple's offspring.
The F1 cross is XHXh x 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 Xh XHXH XHXh XHY XhY XH Y

12 Fig 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 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 aneuploid cell has only 1 copy of a certain chrom. (2n - 1) almost all cases are lethal monosomy X (X0) = Turner’s Syndrome tetrasomy (2n + 2), pentasomy (2n + 3), etc. rare and usually involve only sex chrom. Fig 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 polyploids are 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 mutations in 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 Types of chromosomal mutations

21 Fig. 10. 14. The results of a deletion
Fig 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
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
Fig 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.

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