Chromosomal Basis of Inheritance

Section 10.6 and Chapter 11

I. Chromosomes are the Physical Basis of Mendelian InheritanceA. chromosomal behavior accounts for Mendel's laws and ratios

• movement of chrom. during meiosis Mendel’s laws and ratiosB. chromosome theory of inheritanceC. human chromosomes

1. 22 pairs of autosomal chromosomes• normal body chromosomes

2. 1 pair of sex chromosomes (X and Y)• males: XY; females: XX

D. all chromosomes contain thousands of genesII. Linked Genes

A. genes located on the same chromosome• genes located specifically on sex chromosomes = sex-linked

B. linked genes tend to be inherited together – why?• they do not assort independently

o crosses involving them deviate from Mendel's laws and ratioso parental pheno. are disproportionately represented in offspring

C. even with linked genes, some offspring have traits different from parents• crossing over

Linked genes and their effects on inheritance

A closer look at crossing over

III. Genetic RecombinationA. recombinant offspring

• offspring with traits different from their parentsB. independent assortment

1. Mendel's law2. recombination of unlinked genes

• genes on different chromosomes3. takes place during meiosis I (metaphase)4. yields typical Mendelian ratios

C. crossing over (crossover)1. recombination of linked genes2. takes place during meiosis I (prophase)3. genes located farther apart are more likely to crossover

D. gene mapping1. used to determine the order and position of genes on chrom.2. mapping techniques make use of many chrom. features 3. several different kinds of maps and many uses of gene mapping

A genetic map of a fruit fly chromosome

IV. Sex Chromosomes and Sex-Linked GenesA. sex of any organism has a chromosomal basis

1. varies by type of organism involved2. sex is an inherited trait determined by certain chrom.

a. X-Y system females: XX; males: XYb. X-0 system females: XX; males: Xc. Z-W system females: ZW; males: ZZd. haploid-diploid system females: 2n; males: n

B. sex-linked genes1. have unique patterns of inheritance

a. sex chrom. carry most genes related to sexb. also carry genes unrelated to sex

2. almost all sex-linked genes are carried only on the X-chrom.a. no corresponding gene on the Yb. thus, sex-linked traits are said to be “X-linked”

i. males have only one copy of these X-linked genes – why?ii. females have 2 copies of these X-linked genes – why?

• females can be heterozygous for any X-linked trait, whereas males cannot be

3. several X-linked genes are the cause of sex-linked disordersa. most of these disorders are recessive

• found much more frequently in males than in females – why?o females must inherit 2 copies of the recessive gene, while

males need only inherit one – why?4. sex-linked genetics problems (see handout)

a. hemophilia: X-linked recessive trait, causes blood to clot improperlyb. H = normal allele; h = hemophilia allelec. genotypes:

i. XHXH and XHXh = normal female • XHXh = carrier female

ii. XhXh = hemophiliac femaleiii. XHY = normal maleiv. XhY = hemophiliac male

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.

o hemophiliac man = XhY (by definition)

o normal man the daughter meets = XHY (by definition)

o normal daughter = she must be XHXh, regardless if her mother was XHXH or XHXh

Check this with Punnett Squares:

• Mother XHXH

• Mother XHXh

XHXh XHXh

XHY XHY

XH XH

Xh

Y

XHXh XhXh

XHY XhY

XH Xh

Xh

Y

• Part 2: Determine the possibilities for the couple's offspring.

o The F1 cross is XHXh x XHY

• Part 3: State these possibilities as probabilities.

o normal daughter (XHXH or XHXh) = 50% = 1/2 = (1 in 2) carrier daughter (XHXh) = 25% = 1/4 = (1 in 4)

o hemophiliac daughter (XhXh) = 0%

o normal son (XHY) = 25% = 1/4 = (1 in 4)

o hemophiliac son (XhY) = 25% = 1/4 = (1 in 4)

o chance of 3 sons being hemophiliacs (use Rule of Multiplication):

1/4 x 1/4 x 1/4 = 1/64 = 1.6 % chance

XHXH XHXh

XHY XhY

XH Xh

XH

Y

Fig. 11.16 X-linked inheritance

V. Errors in Chromosomal InheritanceA. genetic disorders can be caused by:

1. recessive alleles on any chromosome, esp., X-linked recessives2. physical/chemical disturbances that damage chrom. or alter inheritance3. errors in meiosis that alter inheritance

B. nondisjunction – an error during meiosis1. can occur in two ways:

a. homologous chromosomes fail to separate (meiosis I)b. sister chromatids fail to separate (meiosis II)

2. one gamete receives two of the same chrom., the other receives no copy• abnormal gamete unites with normal one at fertilization

o aneuploidy

Fig. 10.10 Nondisjunction

3. trisomya. aneuploid cell has a chromosome in triplicate (2n + 1)b. trisomy 21 = Down's Syndromec. trisomy 18 = Edward’s Syndromed. Poly-X (XXX)e. Klinefelter's Syndrome (XXY)f. Jacob’s Syndrome (XYY)

4. monosomya. aneuploid cell has only 1 copy of a certain chrom. (2n - 1)b. almost all cases are lethalc. monosomy X (X0) = Turner’s Syndrome

5. tetrasomy (2n + 2), pentasomy (2n + 3), etc.o rare and usually involve only sex chrom.

Fig. 10.11 A child with Down’s Syndrome. Note the karyotype showing an extra chromosome #21

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

C. polyploidy1. organism possesses more than two complete sets of chrom.

• triploidy (3n) and tetraploidy (4n)2. common in plant kingdom; very rare in animals3. can result from complete nondisjunction during meiosis4. polyploids are more nearly normal than aneuploids – why?

D. mosaicism 1. chrom. abnormalities that do not show up in every cell

• only present in some cells and tissues2. an ind. has two populations of cells with different genotypes

• both came from a single fertilized egg3. usually results from mutations in mitosis, early in embryonic devel.4. symptoms less severe than if all cells are affected

Examples of mosaicism

Blashko Lines

Heterochromia

E. structural alterations of chromosomes1. alterations in the physical/chemical structure of chrom.2. most have harmful effects; but some beneficial 3. deletions4. duplications

• often have beneficial effects major evol. mechanism5. inversions6. translocations

Fig. 10.13 Types of chromosomal mutations

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

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

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.