1. Chromosomes and Human Genetics
Chapter 12

2. Chromosome Theory of Inheritance
Genes have specific locations (loci) on chromosomes.
It is the chromosomes that separate and assort independently during meiosis.
This explains Mendel’s laws of inheritance.

3. Karyotype
A “picture” of chromosomes arrested in metaphase and sorted by length, centromere location or other defining features
Cultured cells are arrested at metaphase by adding colchicine
This is when chromosomes are most condensed and easiest to identify
Used to help answer questions about an individual’s chromosomes
Lets us see sex chromosomes and look for abnormalities in sex chromosomes or autosomes
XX (or XY)

4. Linkage is the tendency of genes located on the same chromosome to be transmitted to­gether in inheritance.
Linkage can be disrupted by crossing over—the exchange of parts of homologous chromosomes.
Certain alleles that are linked on the same chromosome tend to remain together during meiosis because they are positioned closer together on the chromosome.
This eventually led to the generalization that the probability that a cross over will disrupt the linkage of two genes is proportional to the distance that separates them.
The careful analysis of recombination patterns in experimental crosses has resulted in linkage mapping of gene locations.

5. Crossover Frequency Proportional to the distance that separates genes
The closer the genes are to each other, the less likely crossing over will occur A B C D
Crossing over will disrupt linkage between A and B more often than C and D
In-text figure Page 201

6. Sex Chromosomes Discovered in late 1800s Mammals, fruit flies
XX is female, XY is male
Butterflies, moths, some birds and some fish
XX is male, XY female
Human X and Y chromosomes function as homologues during meiosis
Differ physically, one is shorter (Y)
Differ in the genes they carry

7. female
(XX)
male
(XY)
Sex Determination
eggs
sperm
Male gamete determine sex of offspring in mammals and fruit flies Y X X x XX XY X Y
Figure 12.5 Page 198

8. The Y Chromosome The X Chromosome
Fewer than two dozen genes identified
One is the master gene for male sex determination
SRY gene (sex-determining region of Y)
SRY present, testes form
SRY absent, ovaries form
The X Chromosome
Carries more than 2,300 genes
Most genes deal with nonsexual traits
Genes on X chromosome can be expressed in both males and females

9. Sex- Linkage homozygous dominant female recessive male Gametes: x X X
All F1
have red eyes
Gametes: x X X X Y
1/2
1/2
1/4
1/2
1/2
1/4
1/4 F2
generation:
1/4
Figure 12.7 Page 200

10. X-Linked Recessive Inheritance
The characteristics of this condition are:
The mutated gene occurs only on the X chromosome.
Heterozygous females are phenotypically normal; males are more often affected because the single recessive allele (on the X chromosome) is not masked by a dominant gene.
A normal male mated with a female heterozygote have a 50 percent chance of producing carrier daughters and a 50 percent chance of producing affected sons. In the case of a homozygous recessive female and a normal male, all daughters will be carriers and all sons affected.

11. X-Linked Recessive Inheritance
Males show disorder more than females
Son cannot inherit disorder from his father
Figure 12.12a Page 205

12. Examples of X-Linked Traits
Color blindness
Inability to distinguish among some of all colors
Hemophilia
Blood-clotting disorder
1/7,000 males has allele for hemophilia A
Was common in European royal families

13. Fragile X Syndrome An X-linked recessive disorder
Causes mental retardation
Mutant allele for gene that specifies a protein required for brain development
Allele has repeated segments of DNA

14. Autosomal Recessive Inheritance
1. The characteristics of this condition are:
a. Either parent can carry the recessive allele on an autosome.
b. Heterozygotes are symptom-free; homozygotes are affected.
c. Two heterozygous parents have a percent chance of producing heterozygous children and a 25 percent chance of producing a homozygous recessive child. When both parents are homozygous, all children can be affected.
2. Human examples: CF, PKU, Tay Sachs

15. Autosomal Recessive Inheritance Patterns
If parents are both heterozygous, child will have a 25% chance of being affected
Figure 12.10a Page 204

16. Autosomal Dominant Inheritance
The dominant allele is nearly always expressed and if it reduces the chance of surviving or repro­ducing, its frequency should decrease; mutations, nonreproductive effects, and post­reproductive onset work against this hypothesis.
If one parent is heterozygous and other homozygous recessive, there is a 50 percent chance that any child will be heterozygous.
Human examples: Huntingddon’s Disease (nervous system deterioration, death, Symptoms don’t usually show up until person is past age 30, People often pass allele on before they know they have it) and Achondroplasia (a form of dwarfism)

17. Autosomal Dominant Inheritance
Trait typically appears in every generation
Figure 12.10b Page 204

18. Pedigrees A chart of genetic connections among individuals
A graphic representation of a families traits
Provides data on inheritance patterns through several generations.
Knowledge of probability and Mendelian inheritance patterns is used in analysis of pedigrees to yield clues to a trait's genetic basis.

19. Pedigree Symbols male female marriage/mating
offspring in order of birth,
from left to right
Individual showing trait being studied
sex not specified
I, II, III, IV...
generation
Figure 12.9a Page 202

20. Pedigree for PolydactylyII
III IV V
female
male
5,5 6,6 *
5,5 6,6
6,6 5,5
6,6 5,5 6 7
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,6 6,7 12
6,6 6,6
*Gene not expressed in this carrier.
Figure 12.9b Page 202

21. Changes in Chromosome Structure
Portions of chromosomes may be lost or rearranged.
Results in chromosomal mutations.
4 types:
Duplications, deletions, inversions or translocations

22. Gene sequence that is repeated several to hundreds of times
Duplication
Gene sequence that is repeated several to hundreds of times
normal chromosome
one segment
repeated
three repeats

23. Inversion A linear stretch of DNA is reversed within the chromosome
alters the position and sequence of the genes so that gene order is reversed.
segments
G, H, I become inverted
In-text figure Page 206

24. Translocation
A piece of one chromosome becomes attached to another nonhomologous chromosome a part of one chromosome is transferred to a nonhomologous chromosome
Most are reciprocal
In-text figure Page 206
one chromosome
a nonhomologous
chromosome
nonreciprocal translocation
In-text figure Page 206

25. Deletion Loss of some segment of a chromosome
Most are lethal or cause serious disorder

26. Changes in Chromosome Number
Nondisjunction
Failure of chromosomes to separate properly
Results in gametes or daughter cells with abnormal number of chromosomes.
If a gamete with an extra chromosome (n + 1) joins a normal gamete at fertilization, the diploid cell (zygote )will be 2n + 1; this condition is called trisomy.
If an abnormal gamete is missing a (n - 1) joins a normal gamete at fertilization, the diploid cell (zygote )will be 2n – 1; this condition is called monosomy.

27. nondisjunction at anaphase I
chromosome alignments at metaphase I
nondisjunction at anaphase I
alignments at metaphase II
anaphase II
Page 208

28. Aneuploidy
Individuals have one extra or less chromosome
(2n + 1 or 2n - 1)
Major cause of human reproductive failure
Most human miscarriages are aneuploids
Polyploidy
Individuals have three or more of each type of chromosome (3n, 4n)
Common in flowering plants
Lethal for humans
99% die before birth
Newborns die soon after birth

29. Changes in the Number of Sex Chromosomes
Changes in the Number of Autosomes
Down Syndrome - Trisomy of chromosome 21
Changes in the Number of Sex Chromosomes
Turner Syndrome - females whose cells have only one X chromosome (designated XO). (monosomy)
XXX Syndrome
Klinefelter Syndrome - Nondisjunction results in an extra X chromosome in the cells (XXY)
XYY Condition -Extra Y chromosome in these males (does not affect fertility)

30. Chi-Square statistical analysis of genetics data
X2 = ∑ (o-e)2 e
o = observed numbers
e = expected numbers
Solve the equation
Find the degrees of freedom (one less than the total number of choices – types of offspring phenotypes)
Look at the critical value table
if your answer is less than .05 value, then your results deviate from the expected only from chance
If your answer is greater than the .05 value, then your results deviated from the expected for some other reason and you have to figure out why