Date post: | 30-Mar-2018 |
Category: |
Documents |
Upload: | trinhxuyen |
View: | 215 times |
Download: | 0 times |
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.
• Non-genetics phenotypic expression: environmental effect and epigenetics
These are sex chromosomesThe chromosome pairs 1 trough 22 are autosome
Karyotype; display of condensed human chromosomes
Patterns of InheritanceGregor Mendel
- Studied variation in plants, patterns of inheritance in garden peas
- Used math to explain biological phenomena
TerminologyCharacter 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 started with True-breeding plants
F1 generationF2 generationF2 ratio
Purple flower- dominant trait
White flower- recessive trait
Mendel was looking for a model that can account for the 3:1 ratio that he observed in the F2 generation
Mendel’s Model1. An organism inherits two alleles (one
from each parent).2. One allele is dominant and the other is
recessive3. Mendel’s first law of segregation: the
two alleles segregate (separate) during gamete formation
4. Mendel’s second law of independent assortment
Vocabulary used in GeneticsAn 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.
Eye ColorWild-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
• 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 Disordershttp://www.ygyh.org/
Tay-SachsSickle Cell DiseaseCystic FibrosisHuntington 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 and orientation
Germ cells in testes (XY) produce sperms withX: 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 TraitsPossible genotypes
XAXA −Homozyogus wild-type femaleXAXa −Heterozygous female carrierXaXa −Homozygous mutant female
XAY − Hemizygous wild-type maleXaY− 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 dystrophyhttp://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 chromosomesSex 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/BiologyPages/S/SexChromosomes.html
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 InactivationA 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 testesc. male fertility
http://ghr.nlm.nih.gov/chromosome=Y
Factors Affecting Phenotypic Expression
Genetic characteristics inherited in a Mendelian fashion
1. Incomplete dominance 2. Multiple alleles- co-dominance3. Pleiotropy4. Polygenic inheritance
Non genetic factors affecting phenotype1. Environmental effect2. Epigenetic factors
Genotypes:HH
Homozygousfor ability to make
LDL receptors
HhHeterozygous
hhHomozygous
for inability to makeLDL receptors
Phenotypes:
LDLLDLreceptor
Cell
Normal Mild disease Severe disease
Incomplete dominance affects severity of disease
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 homozygousfor sickle-cell allele
Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped
Sickle-cell (abnormal) hemoglobin
Sickle cells
Breakdown ofred blood cells
Clumping of cellsand clogging of
small blood vessels
Accumulation ofsickled cells in spleen
Physicalweakness
Anemia Heartfailure
Pain andfever
Braindamage
Damage toother organs
Spleendamage
Impairedmental
functionParalysis Pneumonia
and otherinfections
Rheumatism Kidneyfailure
Quantitative characters show additive effect of multiple genes, e.g skin color and height in humans
4. Polygenic inheritance
Phenotype of Hydrangea flower color
• Blue flowers in highly acid soil• Pink flowers in neutral or slightly acid soil
Non-genetic factors affecting phenotype: Environmental effects
Non-genetic factors affecting phenotype: Epigenetic factors
Gene expression is impacted by changes of chromatin structure through chemical modification
• 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