Patterns of Gene Inheritance

1
Patterns of Gene Inheritance
2
Genetics is the study of genes
What is a gene? A. A factor that controls a
heritable characteristic B. Something on a
chromosome C. Information stored in a segment of
DNA D. Something that encodes a protein
3
Gregor Mendel
4
LOTS O DEFINITIONS

• Gene---A piece of DNA that encodes a particular
  trait. EX a gene for eye-color
• Allelean alternate form of a gene. EXallele for
  blue eyes and allele for brown eyes
• Phenotypethe physical expression of a gene or
  allele. Ex. Blue eyes
• Genotypethe genetic composition of an
  individual.

5
MORE DEFINITIONS

• Locusthe location of a gene on a chromosome.
  Pluralloci
• Dominant alleleexpressed whether alone or in
  pairs. Symbolized by a capital letter. EX Brown
  eye alleleB
• Recessive alleleexpressed only in the absence of
  a dominant allele. Symbolized by lowercase. EX
  blue eye alleleb
• What is the maximum of alleles a diploid
  individual can have at any given locus?

6
STILL MORE DEFINITIONS!!!

• Genotypethe genetic composition of an individual
• Homozygous----containing a pair of the same
  alleles. Can be
• Homozygous recessivetwo recessive alleles EX.
  bb, or
• Homozygous dominant two dominant alleles EX BB
• Heterozygouscontaining two different alleles. EX
  Bb

7
Law of Segregation

• Each individual has two factors (called genes
  today) for each trait.
• Factors segregate during gametogenesis. Ploidy
  level of gametes?
• Fertilization gives each new individual two
  factors again.
• Ploidy level after fertilization?

8
Gene locus
Defs
9
Example--Widows Peak
10
Gametogenesis

• Homologous pairs separate during meiosis. When?
• ? a gamete has only one allele from each pair of
  alleles.
• ? If the primary spermatocyte is genotype Ww, the
  sperm cell would contain either a W or a w, but
  not both.
• Ww represents the genotype of an individual.
• Gametes are represented by W or w.

11
One-Trait Crosses
GENOTYPE?
GENOTYPE?
GENOTYPE?
12
Punnett SquareEXAMPLE 1A Monohybrid Cross
13
Monohybrid cross
14
The One-Trait Testcross
QUESTION How can we determine the genotype of a
dominant phenotype?
Testcross cross the dominant phenotype (unknown
genotype) with the recessive phenotype (known
genotype).
15
The Law of Independent Assortment

• Law of Segregation involves 1 character. What
  about 2 (or more) characters?
• Each pair of alleles segregates independently of
  the other pairs
• All possible combinations of alleles can occur in
  the gametes
• EX two pairs of alleles segregate independently
  of each other.
• When will different alleles not sort
  independently?

16
Independent Assortment
Alignment of Homologs at Metaphase I
Telophase II
17
Segregation and Independent Assortment
18
MEIOSIS GENETIC VARIATION

• Independent Assortment
• Diploid organisms can produce 2n diff. gametes
• Ea. homologous pair can orient in two different
  ways
• Humans 223 8,388,608
• (est. 8.4 million)

Each homologous pair can orient in either of two
ways
19
HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT
Eye color
Hair color
Gene for browneyes
Gene for blueeyes
r
Gene for black hair
Gene for red hair
20
INDEPENDENT ASSORTMENT
During meiosis I, tetrads can line up 2n
different ways.
OR
Meiosis I II
Brown eyesBlack hair
Blue eyesRed hair
Blue eyesBlack hair
Brown eyesRed hair
21
Linked Genes and Independent Assortment
L
l
L
L
Alignment of Homologs at Metaphase I
l
22
Dihybrid cross
What are the possible genotypes of this
individuals gametes?
Probabilities
23
Two-trait Testcross-How?
24
Genetic Disorders

• Pedigree charts- show patterns of inheritance

25
Autosomal Dominant Disorders

• Autosomes---Non-sex chromosomes
• Dominant allele
• ?when under what genotype(s)will an individual
  show the disorder?

26
Examples Autosomal Dominant Disorder

• Neurofibromatosis
• Small benign tumors,
• Gene on chromosome 17.
• Huntington Disease
• Progressive degeneration of the nervous system
• Normal until middle age
• No treatment
• Gene has been isolated (chromosome 4), genetic
  testing can diagnose

27
Huntington disease
28
Autosomal recessive pedigree chart

• Autosomes---Non-sex chromosomes
• Recessive allele
• ?when under what genotype(s)will an individual
  show the disorder?

29
Autosomal Recessive Disorders

• Tay-Sachs Disease
• Common among United States Jews of central and
  eastern European descent.
• Death by the age of three or four.
• Defective enzyme in lysosomes.
• Cystic Fibrosis
• Most common lethal genetic disorder among
  Caucasians
• Defective chloride ion transport protein
• Osmotic imbalance results in thick mucous in
  lungs and pancreatic ducts

30
Cystic fibrosis therapy
31
Phenylketonuria (PKU)

• allele on chromosome 12.
• lack an enzyme needed for metabolism of
  phenylalanine (an amino acid)
• Urine test diagnostic.
• Brain damage unless controlled by diet.

32
Polygenic Inheritance

• Polygenic (Quantitative) Traits
• Governed by more than one gene pair.
• Several genes determine the phenotype.
• Produce bell-shaped curve.
• EX Skin color

33
EX 2 Polygenic Inheritance
34
Polygenic Disorders

• Examples cancer, schizophrenia, hypertension,
  diabetes, etc
• several genes involved
• also environmental influences.

35
Multiple Allelic Traits

• Often more than two alleles exist for a
  particular gene locus.
• Maximum of alleles at any given locus?
• Each individual inherits only two alleles for
  these genes!!!
• EX Human Blood type

36
Inheritance of blood type
Details
37
Incompletely Dominant Traits

• Codominance--both alleles equally expressed in a
  heterozygote. EX ABO Blood type
• Incomplete dominance--heterozygote shows
  intermediate phenotype, representing a blending
  of traits. EX Wavy hair in Caucasians
• Phenotypic ratio ?
• 1 2 1.

38
Incomplete dominance
39
Example 2-Incomplete Dominance
40
Sickle-Cell Disease

• Caused by incompletely dominant alleles.
• Single nucleotide mutation causes abnormal
  hemoglobin.
• HbA normal hemoglobin
• HbS sickled condition.
• Genotype of normal?
• Genotype of sickled homozygote?
• Genotype of intermediate phenotype?

41
Sickle-Cell Disease
42
Molecular Explanation-Dominance
Lets Get Small!!!

• Simple Dominance
• One-half the amount of gene product sufficient
  for phenotype
• Incomplete Dominance
• Recessive allele not expressed in heterozygote
• Homozygous dominant 2 doses, full phenotype
• Heterozygote1 dose, reduced phenotype
• Codominant
• Both alleles expressed, ?combined phenotype

43
Everythings Relative
44
WHATS HAPPENING??
45
SUMMARY

• Physical Basis of Genes
• Lots o Definitions
• Law of Segregation
• Law of Independent Assortment
• One-Trait Crosses
• Law of Independent Assortment
• Two-Trait Crosses
• Genetic disorders
• Polygenic Traits
• Incomplete Dominance/Codominance

46
REMEMBER!!!

• Relate genetics to meiosis
• For genetics problemsfirst try to figure the
  genotype(s) of the gametes