Download Sex-linked and Mitochondrial Inheritance (Learning Objectives

Sex-linked and Mitochondrial Inheritance (Learning Objectives)
• Explain how gender is determined in mammals.
• Define X- or Y-linked genes. How does the location of a gene on the
X chromosome affect its gender-related transmission?
• Use a Punnett square to determine the probability of passing of an Xlinked gene and the phenotype to girls or boys based on the
genotypes of the parents.
• Explain the difference between sex-limited traits and sex-influenced
traits.
• Explain X-inactivation and why it exists only in cells of females.
• Explain the functions of the Y chromosome gene and the pattern of
inheritance of Y-linked traits.
• Explain the pattern of inheritance of genes present on the
mitochondrial DNA.
Gender
• Maleness or femaleness is determined at
conception
• Another level of sexual identity comes from the
control that hormones exert on development
• Finally, both psychological and sociological
components influence sexual feelings
During the fifth week of
prenatal development, all
embryos develop two sets of:
- Unspecialized (indifferent)
gonads
- Reproductive ducts:
Müllerian (female-specific) &
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)
Figure 6.1 Figure 6.1
Sex determination in Mammals:
the X-Y system
Karyotype designation: 46, XY (male)
46, XX (female) (homogametic)
Males are heterogametic
Germ cells in testes (XY) produce sperms with
X: 50%
Y: 50%
Females are homogametic
Germ cells in ovaries (XX) produce only
X eggs
Sex Determination in Humans
Figure 6.6
Figure 6.6
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
- Many repeated DNA segments
Figure 6.2
Anatomy of the Y Chromosome
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)
Figure 6.3
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 dihydrotesterone (DHT)
hormones are secreted and stimulate male
structures
The inheritance of genes of X chromosome
• males have only a single X chromosome
• almost all the genes on the X have no
counterpart on the Y
• Genes are described as sex-linked or Xlinked.
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
X-linked Recessive Traits
Examples:
- Ichthyosis = Deficiency of an enzyme that removes
cholesterol from skin
- Color-blindness = Inability to see red and green
colors
http://www.biology.arizona.edu/human_bio/problem_sets/color_bli
ndness/color_blindness.html
- Hemophilia = Disorder of blood-clotting
http://www.ygyh.org
Ichthyosis
Figure 6.7
Figure 6.7
X-linked Dominant Traits
Congenital
generalized
hypertrichosis
Figure 6.10
Sex-Limited Traits
Traits that affect a structure or function occurring
only in one sex
The gene may be autosomal or X-linked
Examples:
- Beard growth
- Milk production
- Preeclampsia in pregnancy
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
The gene may be autosomal or X-linked
Example:
- Pattern baldness in humans (autosomal)
- A heterozygous male is bald, but a
heterozygous female is not
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
X Inactivation
X inactivation occurs early in prenatal
development
It is an example of an epigenetic change
The XIST gene on the inactive X encodes an
RNA that binds to and inactivates the X
chromosome
Figure 6.11
Figure 6.12
X Inactivation
A female that expresses the phenotype
corresponding to an X-linked gene is a
manifesting heterozygote (calico cats)
Figure 6.12
Y-linked genes
The Y chromosome in males has 231 gene genes
whose protein products are involved in:
a. control of changing sex of the fetus from female to
male
b. development of male testes
c. male fertility
http://ghr.nlm.nih.gov/chromosome=Y
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
Imprints are erased
during meiosis
- Then reinstituted
according to the
sex of the
individual
Figure 6.13
Mitochondrion
• Organelle providing cellular energy
• Contains small circular DNA called mtDNA
- 37 genes without noncoding sequences
• No crossing over and little DNA repair
• High exposure to free radicals
• Mutation rate is greater than nuclear DNA
• A cell typically
has thousands of
mitochondria,
and each has
numerous copies
of its “minichromosome”
Figure 5.8
Mitochondrion
• Mitochondrial genes are transmitted from
mother to all of her offspring
Figure 5.7
Mitochondrial Disorders
Mitochondrial genes encode proteins that participate in
protein synthesis and energy production
Several diseases result from mutations in mtDNAaternally
inherited
Examples:
- Mitochondrial myopathies – Weak and flaccid muscles
- Leber optical atrophy – Impaired vision
Ooplasmic transfer technique can enable woman to avoid
transmitting a mitochondrial disorder
Heteroplasmy
• The mtDNA genome sequence may not the
same in all mitochondria
• The phenotype reflects the proportion of
mitochondria bearing the mutation
Figure 5.9