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1
Human Genetics Concepts and Applications
Tenth Edition
RICKI LEWIS
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
PowerPoint® Lecture Outlines Prepared by Johnny El-Rady, University of South Florida
6 Matters of Sex
2
Our Sexual Selves Maleness or femaleness is determined at
conception
Another level of sexual identity comes from the control that hormones exert on development
3
Sexual Development During the fifth week of prenatal development, all
embryos develop two sets of: - Unspecialized (indifferent) gonads - Reproductive ducts – Müllerian (female-specific) and Wolffian (male-specific)
An embryo develops as a male or female based on
the absence or presence of the Y chromosome - Specifically the SRY gene (sex-determining region of the Y chromosome)
4
Sex Chromosomes Determine Gender Human males are the heterogametic sex
with different sex chromosomes, (XY) Human females are the homogametic sex
(XX)
In other species sex can be determined in many ways - For example, in birds and snakes, males are homogametic (ZZ), while females are heterogametic (ZW)
5
X and Y Chromosomes X chromosome
- Contains > 1,500 genes - Larger than the Y chromosome - Acts as a homolog to Y in males
Y chromosome - Contains 231 genes
Figure 6.1
6
Anatomy of the Y Chromosome
Figure 6.2
Pseudoautosomal regions (PAR1 and PAR2) - 5% of the chromosome - Contains genes shared with X chromosome
Male specific region (MSY) - 95% of the chromosome - Contains majority of genes including SRY and AZF (needed for sperm production)
7
SRY Gene Encodes a transcription factor protein Controls the expression of other genes Stimulates male development Developing testes secrete anti-Mullerian
hormone and destroy female structures Testosterone and dihydrotestosterone
(DHT) are secreted and stimulate male structures
8
Abnormalities in Sexual Development
Pseudohermaphroditism = Presence of male and female structures but at different stages of life - Androgen insensitivity syndrome - 5-alpha reductase deficiency - Congenital adrenal hyperplasia = High levels of androgens
9
Androgen insensitivity syndrome
• Androgen insensitivity syndrome = Lack of androgen receptors
• Androgens are male hormones, testosterone and dihydrotestosterone
• Person is XY, but cells don’t receive the signal to develop as male, so the person looks female.
10
5-alpha reductase deficiency
• 5-alpha reductase deficiency = Absence of DHT • XY individual with normal Y chrmosome. • Male reproductive tract develops normally on the
inside. • 5-alpha reductase catalyzes the reaction of
testosterone to form DHT.Without DHT, a penis cannot form.
• At puberty, when the adrenal glands start to produce testosterone, this XY person who though she was female will begin developing male secondary sex characteristics, including a penis.
12
Sex Ratios The proportion of males to females in a
human population Calculated by # of males / # of females
multiplied by 1,000 Primary sex ratio – At conception Secondary sex ratio – At birth Tertiary sex ratio – At maturity Sex ratios can change markedly with age
14
Y-linked Traits
Genes on the Y chromosome are said to be Y-linked
Y-linked traits are very rare
Transmitted from male to male No affected females Currently, identified Y-linked traits involve
infertility and are not transmitted
15
X-linked Traits
Possible genotypes X+X+ - Homozyogus wild-type female X+Xm - Heterozygous female carrier XmXm - Homozygous mutant female
X+Y - Hemizygous wild-type male XmY- Hemizygous mutant male
17
X-linked Recessive Traits
Examples: - Ichthyosis = Deficiency of an enzyme that removes cholesterol from skin
- Color-blindness = Inability to see red and green colors
- Hemophilia = Disorder of blood-clotting - Duchenne Muscular Dystrophy = muscles deteriorate over time
21
Duchenne muscular dystrophy.
• Caused by a mutation in the gene that codes for dystrophin, which is located on the X chromosome.
• Muscles deteriorate over time. • A child with DMD is often wheelchair-bound
by the age of 12. • Survival is increasing into the 20s and 30s.
Previously, these young men died in their teens.
25
Solving Genetic Problems
Steps to follow: 1) Look at the inheritance pattern 2) Draw a pedigree 3) List genotypes and phenotypes and their probabilities 4) Assign genotypes and phenotypes 5) Determine how alleles separate into gametes 6) Use Punnett square to determine ratios 7) Repeat for next generation
26
Sex-Limited Traits
Traits that affect a structure or function occurring only in one sex but can be passed on by either sex.
The gene may be autosomal or X-linked
Examples: - Beard growth - Milk production - Preeclampsia in pregnancy
27
Sex Limited traits
• Beard growth—only men grow beards, but women can pass on a gene for heavy beard growth to their sons.
• Men can also pass on genes that affect milk production in daughters even though they themselves don’t produce milk.
• Preeclampsia—sudden spike in blood pressure late in pregnancy.
• Placental development can be affected by father’s genes.
28
Sex-Influenced Traits
Traits in which the phenotype expressed by a heterozygote is influenced by sex
Allele is dominant in one sex but recessive in the other
Example: - Pattern baldness in humans
- A heterozygous male is bald, but a heterozygous female is not
29
X Inactivation
Females have two alleles for X chromosome genes but males have only one
In mammals, X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell
The inactivated X chromosome is called a Barr body
30
X Inactivation
X inactivation occurs early in prenatal development
It is an example of an epigenetic change - An inherited change that does not alter the DNA base sequence
The XIST gene encodes an RNA that binds to and inactivates the X chromosome
32
X Inactivation A female that expresses the phenotype
corresponding to an X-linked gene is a manifesting heterozygote
X inactivation is obvious in calico cats Figure 6.10
33
Genomic Imprinting
The phenotype of an individual differs depending on the gene’s parental origin
Genes are imprinted by an epigenetic event:
DNA methylation - Methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex
34
Imprints are erased during meiosis - Then reinstituted according to the sex of the individual
Figure 6.11
35
Importance of Genomic Imprinting Function of imprinting isn’t well understood,
but it may play a role in development
Research suggests that it takes two opposite sex parents to produce a healthy embryo - Male genome controls placenta development - Female genome controls embryo development
Genomic imprinting may also explain incomplete penetrance
36
Imprinting and Human Disease
Two distinct syndromes result from a small deletion in chromosome 15 - Prader-Willi syndrome - Deletion inherited from father - Angelman syndrome - Deletion inherited from mother