Fig. 15.11
I.
Linkage and Recombination
B.
Recombination
•
•
Possible to use recombination frequencies to
construct genetic map (linkage map) of genes
on chromosome
If two loci are sufficiently far apart, possible to
get double crossing over
Fig. 15.12
II.
Sex Chromosomes and Gender
•
Many systems of sex determination
•
•
A.
Most animals have sex chromosomes
One gender typically homogametic, the other
heterogametic
Humans
•
•
•
Males and females with 22 pairs of autosomes
(do not determine gender)
Question: Is female gender in humans
determined by presence of two X chromosomes
or absence of Y chromosome?
Approach: Examine people with abnormal sex
chromosomes
II.
Sex Chromosomes and Gender
A.
Humans
•
XXY – Klinefelter Syndrome
•
•
Nearly normal males with underdeveloped testes
XO – Turner Syndrome
•
•
•
Phenotypically female with underdeveloped ovaries
YO – Embryo doesn’t develop
Conclusions
1)
2)
•
X chromosome required for development
Genes on Y chromosome determine gender
X and Y chromosome aren’t homologous but have short,
homologous pairing regions that permit synapsis during
meiosis
Sperm containing X and Y chromosomes produced in
equal numbers
•
•
More male babies conceived, die before birth, born (1.06:1)
II.
Sex Chromosomes and Gender
B.
Other Species
1.
X-Y system
•
•
2.
3.
4.
XX = Female, XY = Male
Used by humans
X-O system
Z-W system
Haplo-diploid system
1.
X-Y system
•
•
2.
X-O system (crickets)
•
•
3.
Egg carries X
Sperm carries X or nothing
Z-W system (birds, fishes,
butterflies, moths)
•
•
4.
Egg carries Z or W
Sperm carries Z
Haplo-diploid system (ants)
•
•
Fig. 15.6
XX = Female, XY = Male
Used by humans
Females from fertilized ova
Males from unfertilized ova
II.
Sex Chromosomes and Gender
C.
Sex-Linked Genes
•
•
•
X chromosome in humans contains many genes
required by both males and females
Y chromosome contains fewer genes, mostly
related to “maleness” (testicular development,
affinity for monster trucks, etc.)
Mutations on X chromosome can lead to
genetic disorders (X-linked)
•
•
P: Wild type female, red eyes
Mutant male, white eyes
•
F1: All with red eyes
•
•
F2: Females with red eyes
Half of males with white eyes
Fig. 15.4
II.
Sex Chromosomes and Gender
C.
Sex-Linked Genes
•
Females
•
•
•
•
Inherit one X from mother and one from father
Dominant traits expressed, recessive traits not
Heterozygous – Can be carriers
Males
•
•
•
Inherit all X-linked genes from mother
All X-linked alleles typically expressed
Hemizygous – Can’t be carriers
Fig. 15.7
II.
Sex Chromosomes and Gender
C.
Sex-Linked Genes
•
Disorders
1)
Color blindness
•
Most common in men
2) Duchenne muscular dystrophy
•
Absence of key muscle protein (dystrophin)
3) Hemophilia
•
Absence of protein(s) required for blood clotting
II.
Sex Chromosomes and Gender
D.
X Inactivation
•
Females have two copies of X chromosome
•
•
Fruit flies – Males make single X more active than
either female X
Mammals – One X typically inactivated at random in
each cell (dosage compensation)
•
Barr body – Inactivated X, visible during
interphase as dark area of highly condensed
chromatin
•
Inactivation incomplete; some genes expressed
•
Heterozygous female may express traits from
each X chromosome in ~50% of cells
•
Ex: Calico and tortoiseshell cats
Fig.
15.8
II.
Sex Chromosomes and Gender
E.
Sex-Influenced Genes
•
Some traits inherited autosomally but influenced
by gender
•
•
Male & female with same genotype, different
phenotypes
Ex: Pattern baldness
•
•
•
•
•
Proposed that single pair of alleles determines pattern
baldness – Dominant in males, recessive in females
B1 = Pattern baldness, B2 = Normal hair growth
B1B1 = Pattern baldness in males & females
B1B2 = Pattern baldness in males, normal hair in
females
B2B2 = Normal hair in males & females
III. Chromosomal Abnormalities
A.
Chromosome Number
•
•
•
•
•
Usually due to nondisjunction (chromosomes
fail to separate during anaphase of meiosis)
One gamete receives an extra chromosome,
the other receives one fewer than normal
Condition = aneuploidy
Nondisjunction during mitosis may lead to clonal
cell lines with abnormal chromosome counts
Nondisjunction during meiosis may lead to
gametes (and offspring) with abnormal
chromosome counts
Fig.
15.13
III. Chromosomal Abnormalities
A.
Chromosome Number
1.
Possible outcomes
a.
b.
c.
Trisomy – 2n+1 chromosomes in fertilized egg
Monosomy – 2n-1 chromosomes in fertilized egg
Polyploidy – 3n, 4n, 5n, 6n, etc. chromosomes in
fertilized egg
•
Triploidy – 3n chromosomes
•
Tetraploidy – 4n chromosomes
III. Chromosomal Abnormalities
A.
Chromosome Number
2.
Possible outcomes
•
•
a.
Autosomal aneuploidies highly detrimental and rare
No known autosomal monosomies (100% lethal)
Down’s Syndrome
•
Trisomy of chromosome 21
•
Mental retardation, heart defects, susceptibility to
diseases
•
Affects ca. 1 of every 700 children born in US
•
Frequency increases with age of mother
Fig. 15.15
III.
Chromosomal Abnormalities
A.
Chromosome Number
2.
Possible outcomes
•
b.
c.
d.
e.
Sex chromosome aneuploidies less rare, perhaps due to
dosage compensation and few genes on Y
Klinefelter Syndrome (XXY)
•
Phenotypically male but with Barr bodies
•
Tend to be tall with female-like breasts and reduced
testes
•
May show signs of mental retardation
XYY
•
Phenotypically male but often very tall
•
May have severe acne
XXX
•
Phenotypically normal female
Turner Syndrome (XO)
•
Phenotypically female with no Barr bodies
•
Usually with undeveloped reproductive structures
III. Chromosomal Abnormalities
B.
Chromosome Structure
•
Often results from breakage of chromosomes
and errors in repair
Fig. 15.14
III. Chromosomal Abnormalities
B.
Chromosome Structure
1.
Deletion
•
2.
Cri du Chat Syndrome
•
Deletion of part of short arm of chromosome 5
•
Mental retardation, small head, cry like a kitten
Translocation
•
•
Down’s syndrome may be caused not by trisomy but
by extra material from chromosome 21 attached to
other, large chromosome
Reciprocal translocation between chromosomes 9 and
22 can increase likelihood of developing chronic
myelogenous leukemia (CML)
Fig.
15.16
IV. Exceptions to Mendelian Inheritance
A.
Genomic Imprinting
•
•
Expression of phenotype affected differently by
inheritance of allele from mother vs. father
Imprinting may lead to expression of maternal
or paternal allele for a particular species and
gene
Fig. 15.17