Genetics II

1
Genetics II
2
Mendelian Genetics

• Complete dominance

3
Non-mendelian Genetics

• Codominance
• Incomplete dominance
• Multiple alleles
• Pleiotropy
• Epistasis
• Polygenic inheritance
• Sex-related inheritance

4
Pleiotropy (pleion, more)

• Ability of a gene to affect an organism in
  multiple ways
• Examples
• 40 of cats with white fur and blue eyes are deaf
• Phenylketonuria in humans (PKU)
• Lack of an enzyme that converts Phe ? Tyr
• Mental retardation, reduced hair and skin
  pigmentation, eczema
• Sickle-cell anemia
• Problem with hemoglobin molecules ? misshapen
  RBCs at low O2 concs ? kidney and bone damage

5
(No Transcript)
6
EpistasisOne gene affects expression of another

• Effects of one gene are modified by one or
  several other genes called modifier genes
• Gene whose phenotype is expressed ? epistatic
• Examples
• Coat color in mammals
• Color of wheat grains
• Fruit color in squash
• Bombay phenotype in ABO blood groups in man

7
(No Transcript)
8
Polygenic inheritanceAdditive effects of two or
more genes on a phenotypic character

• Quantitative characters vary in a population
  along a continuum or gradation
• Due to the presence of contributory (ABC) and
  non-contributory alleles (abc)
• Expression can be affected by environmental
  factors

• Example
• Skin pigmentation controlled by at least three
  separately inherited genes
• Each dark skin allele contributes a unit of
  darkness to the individual
• AABBCC ? very dark
• aabbcc ? very light

9
(No Transcript)
10
Polygenic Inheritance Problem Solving

• Petal length of a plant ranges from 4mm to 12mm
  to 20mm. Out of 770 plants, only 3 of them have
  4mm petals.
• Give one genotype for a plant with 12 mm petals.
• Give two possible genotypes for plants with 6 mm
  petals
• What proportion of plants have 14 mm petals?
• What is the phenotype of plants with 7
  contributory alleles?
• How many contributing alleles does a plant with
  4mm petals have?

11
Sex-related inheritanceexpression of traits is
affected by the sex of the individual

• sex is an inherited phenotypic character
  determined by the presence/absence of certain
  chromosomes
• SRY (sex-determining region of Y) in humans
• Sex-linked inheritance
• Sex-limited inheritance
• Sex-influenced inheritance

12
Sex-linked inheritance
(1) Father affected with the disorder Xa, Mother
is homozygous dominant

• Gene linkage genes located on the same
  chromosome are inherited together
• Sex-linkage
• Sex chromosomes contain genes for many
  characters unrelated to sex
• X-linked/Y-linked gene

(2) Carrier mates with a male of normal phenotype
(3) Carrier mates with a male with the disorder
13
Examples of X-linked alleles

• Recessive alleles
• Color-blindness
• Duchenne muscular dystrophy
• Hemophilia
• Testicular feminization

• Dominant traits
• Hypophosphatemia

14
(No Transcript)
15

• Mammalian females
• One of the two X chromosomes is randomly
  inactivated
• If female is heterozygous for a gene located on
  the X chromosome, she is a mosaic

16
Examples of Y-linked (Holandric) Inheritance

• Transmission of genes from father to son
• Testis-determining factor (TDF/SRY gene)
• hypertrichosis

17
Sex-limited inheritance

• involves autosomal genes that are expressed only
  in either males or females
• resulting in a part or function of the body that
  is present in one sex but not the other
• e.g. milk production
• cryptorchidism
• feathers in domestic fowl
• Genotype Female Male
• HH hen-feathered hen-feathered
• Hh hen-feathered hen-feathered
• hh hen-feathered rooster-feathered

18
Sex-influenced inheritance

• dominant in one sex but recessive in the other
• autosomal
• difference in expression due to the hormonal
  difference between the sexes
• in heterozygotes, the expression of the trait is
  affected by sex hormones
• homozygotes unaffected and express the trait
  regardless of the hormone produced

• e.g. pattern baldness
• gene for hair growth pattern has 2 alleles one
  that produces hair all over the head and another
  that causes pattern baldness
• the baldness allele is dominant in males but
  recessive in females
• a heterozygous male is bald, but a heterozygous
  female is not
• Genotype Male (dominant) Female (recessive)
• b1b1 bald bald
• b1b2 bald not bald
• b2b2 not bald
• not bald