Define “karyotype”: autosomal chromosomes and sex ...faculty.sdmiramar.edu/bhaidar/Bio 107...
Transcript of Define “karyotype”: autosomal chromosomes and sex ...faculty.sdmiramar.edu/bhaidar/Bio 107...
Human Genetics (Outline) • Define “karyotype”: autosomal chromosomes and sex chromosomes • Mendel and terminology. • Medelian Pattern of single autosomal gene inheritance; Punnett square • Test cross and purpose. • Pedigrees used to determine the pattern of inheritance and make genetic
predictions. • Gender determination in mammals; X- or Y-linked genes. Pattern of
inheritance of sex-linked genes • Explain X-inactivation in females. • Inheritance of mitochondrial genes • Phenotypic expression and multi-factorial traits Mendelian characters and
provide examples for each: incomplete dominance, co-dominance & multiple allele, pleiotropy, polygenic inheritance, environmental effect.
These are sex chromosomes The chromosome pairs 1 trough 22 are autosome
Karyotype; display of condensed human chromosomes
Patterns of Inheritance Gregor Mendel
- Studied variation in plants, patterns of
inheritance in garden peas - Used math to explain biological phenomena
Terminology Character or characteristic: a heritable
feature e.g. flower color Trait: variant of the character e.g. purple or
white Mendel focused on characters with two
variants “either-or” traits
Mendel had control over which plants he crossed Colored Cotton Campbell video
http://www.dnaftb.org/
dnaftb/1/concept/
Mendel started with True-breeding plants F1 generation F2 generation F2 ratio
Purple flower- dominant trait
White flower- recessive trait
Mendel worked with pea plant characteristics with two traits each
Mendel was looking for a model that can account for the 3:1 ratio that he observed in the F2 generation
Mendel’s Model
1. An organism inherits two alleles (one from each parent).
2. One allele is dominant and the other is recessive
3. Mendel’s first law of segregation: the two alleles segregate (separate) during gamete formation
4. Mendel’s second law of independent assortment
Mendel's First Law – Segregation
A Punnett square predicts the results of a genetic cross between individuals of known genotype.
Vocabulary used in Genetics An organism with two identical alleles is
homozygous for that character. Organisms with two different alleles for a character
is heterozygous for that character. A description of an organism’s traits is its
phenotype. A description of its genetic makeup is its
genotype.
Test Cross
Used to determine the genotype of a dominant trait
Figure 4.9
Mendel's Second Law – Independent Assortment
Mendelian characters of humans
Eye Color Wild-type human eye color is brown - Blue and green eyes stemmed from
mutations that persisted
The surface of the back of the iris contributes to the intensity of eye color
Figure 4.8
Pedigree Analysis
• A pedigree can help us understand the past and to predict the future.
• We can use the normal Mendelian rules,
to predict the probability of specific genotypes and phenotypes.
Examples of Genetic Disorders http://www.ygyh.org/ Tay-Sachs Sickle Cell Disease Cystic Fibrosis Huntington Disease
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
Sex determination in Mammals: the X-Y system
Karyotype designation: 46, XY (male)
46, XX (female)
Germ cells in testes (XY) produce sperms with X: 50% Y: 50% Germ cells in ovaries (XX) produce only X eggs
• The X chromosome have genes for many
characters unrelated to sex • Each conception has about a fifty-fifty chance of
producing a particular sex
The inheritance of genes of X chromosome follows special rules, because:
• males have only a single X chromosome • almost all the genes on the X have no
counterpart on the Y • any gene on the X, even if recessive in
females, will be expressed in males. • Genes are described as sex-linked or X-
linked.
X-linked Recessive Traits Possible genotypes XAXA − Homozyogus wild-type female XAXa − Heterozygous female carrier XaXa − Homozygous mutant female XAY − Hemizygous wild-type male XaY− Hemizygous mutant male
Examples of X-linked Recessive Diseases
Hemophilia A, a blood clotting disorder caused by a mutant gene encoding the clotting factor VIII
Duchenne muscular dystrophy http://www.ygyh.org Color blindness (X-linkage) http://www.biology.arizona.edu/human_bio/proble
m_sets/color_blindness/color_blindness.html
Human Chromosomes Homologous autosomes: 22 pairs = 44 chromosomes Sex chromosomes one pair XX or XY (X and Y share partial homology) Dose of expressed genes?
X-inactivation
In females, only one of the X chromosomes is active. The second is inactivated The inactive X chromosome appears as a condensed
chromosome during interphase (Barr body) http://users.rcn.com/jkimball.ma.ultranet/BiologyPage
s/S/SexChromosomes.html
Figure 6.12
X Y
X XX XY
Xh XhX XhY
Hemophilia A
In XhX heterozygote female, which X is active?
X-inactivation is random: 50% of cells Xh 50% of cells X
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html
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 70 to 200 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
Factors Affecting Phenotypic Expression of Mendelian inherited characteristics
1. Incomplete dominance 2. Multiple alleles- co-dominance 3. Pleiotropy 4. Polygenic inheritance 5. Environmental effect 6. Epigenetic factors
Genotypic ratio same as phenotypic ratio
1. Incomplete dominance
Genotypes: HH
Homozygous for ability to make
LDL receptors
Hh Heterozygous
hh Homozygous
for inability to make LDL receptors
Phenotypes:
LDL LDL receptor
Cell
Normal Mild disease Severe disease
Incomplete dominance affects severity of disease
2. Multiple alleles, the human ABO blood system
Multiple alleles of the ABO blood system - Three alleles, IA, IB, and I.
Both the IA and IB alleles are dominant to the i allele
The IA and IB alleles are co-dominant to each other.
- Because each individual carries two alleles,
there are six possible genotypes and four possible blood types.
3. Pleiotropy - A single gene may affect many phenotypic characteristics involving multiple systems - Sickle cell Disease http://www.ygyh.org/
Individual homozygous for sickle-cell allele
Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Sickle-cell (abnormal) hemoglobin
Sickle cells
Breakdown of red blood cells
Clumping of cells and clogging of
small blood vessels
Accumulation of sickled cells in spleen
Physical weakness
Anemia Heart failure
Pain and fever
Brain damage
Damage to other organs
Spleen damage
Impaired mental
function
Paralysis Pneumonia and other infections
Rheumatism Kidney failure
Quantitative characters show additive effect of multiple genes, e.g skin color and height in humans
4. Polygenic inheritance
5. Environmental effects
Phenotype of Hydrangea flower color
• Blue flowers in highly acid soil • Pink flowers in neutral or slightly acid soil
6. Epigenetic factors Gene expression is impacted by chemical
modification of chromatin • DNA methylation • histone deacetylation
Inheritance of nuclear and mitochondrial or chloroplast DNA
• Nuclear DNA- diploid on linear segmented chromosomes
• Mitochondria and Chloroplasts- single haploid circular chromosome
Replicate independently of nucleus
• Defects in mitochondrial genes are passed through the maternal lineage
During fertilization sperm brings only the nuclear DNA, all mitochondrial DNA is maternal from the egg.
No Mendelian patterns of inheritance Can be used to determine maternal lineage