1. THE CHROMOSOMAL BASIS OF INHERITANCE
9.17 Chromosome behavior accounts for Mendel’s principles
Genes are located on chromosomes
Their behavior during meiosis accounts for inheritance patterns

2. 9.4 Homologous chromosomes bear the two alleles for each characteristic
Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes
GENE LOCI
DOMINANT
allele P a B P a b
RECESSIVE
allele
GENOTYPE: PP aa Bb
HOMOZYGOUS for the dominant allele
HOMOZYGOUS for the recessive allele
HETEROZYGOUS
Figure 9.4

3. The chromosomal basis of Mendel’s principles
Figure 9.17

4. 9.18 Genes on the same chromosome tend to be inherited together
Certain genes are linked
They tend to be inherited together because they reside close together on the same chromosome

5. Figure 9.18

6. 9.19 Crossing over produces new combinations of alleles
This produces gametes with recombinant chromosomes
The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

7. A B a b B A a b A b a B Tetrad Crossing over Gametes
Figure 9.19A, B

8. Figure 9.19C

9. 9.20 Geneticists use crossover data to map genes
Crossing over is more likely to occur between genes that are farther apart
Recombination frequencies can be used to map the relative positions of genes on chromosomes
Chromosome g c l
17% 9%
9.5%
Figure 9.20B

10. SEX CHROMOSOMES AND SEX-LINKED GENES
9.21 Chromosomes determine sex in many species
A human male has one X chromosome and one Y chromosome
A human female has two X chromosomes
Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

11. Parents’ diploid cells
(male)
(female)
Parents’ diploid cells X Y
Male
Sperm
Egg
Offspring (diploid)
Figure 9.21A

12. Other systems of sex determination exist in other animals and plants
The X-O system
The Z-W system
Chromosome number
Figure 9.21B-D

13. 9.22 Sex-linked genes exhibit a unique pattern of inheritance
All genes on the sex chromosomes are said to be sex-linked
In many organisms, the X chromosome carries many genes unrelated to sex
Fruit fly eye color is a sex-linked characteristic
Figure 9.22A

14. Their inheritance pattern reflects the fact that males have one X chromosome and females have two
These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female
Male
Female
Male
Female
Male
XRXR
XrY
XRXr
XRY
XRXr
XrY XR XR Xr Xr XR XR
XRXr Y
XRXR Y
XRXr Y Xr Xr
XRY
XrXR
XRY
XrXr
XRY
XrY
XrY
R = red-eye allele
r = white-eye allele
Figure 9.22B-D

15. 9.23 Connection: Sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles
Examples: hemophilia, red-green color blindness
These are mostly seen in males
A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected
Figure 9.23A

16. Czar Nicholas II of Russia
A high incidence of hemophilia has plagued the royal families of Europe
Queen Victoria
Albert
Alice
Louis
Alexandra
Czar Nicholas II of Russia
Alexis
Figure 9.23B