Download Mendelian Genetics

Lecture 12: Mendelian Genetics
I.
Biological Background
A.
1.
2.
3.
4.
Terminology
Autosomes—express non-sex-based traits
Sex chromosomes—determine genetic sex
Karyotype—diploid chromosomal compliment of all cells
Genome—genetic (DNA) makeup of a cell
a.
Humans have a diploid genome
i.
One copy from egg (maternal copy)
ii.
One copy from sperm (paternal copy)
Chromosome—structure carrying heredity factors
a.
Only visible during cell division
b.
Humans have 23 diploid chromosome (46 total)
i.
Homologous pairs
c.
Chromosomes include many genes
Locus—location
a.
Physical location of a gene on a chromosome
Allele—matched genes at the same locus on homologous chromosomes
a.
Alleles can code for same or alternate forms of a trait
b.
Most genes in diploid organisms have two alleles
c.
Possible number of alleles for a given gene can be greater than two
Homozygous—two alleles controlling a single trait are alike
a.
Pure breeding line
Heterozygous—two alleles controlling a single trait are dissimilar
a.
Character expressed may vary as a function of pattern of inheretance
Dominant—one allele masks the expression of its partner
a.
Nomenclature—capital letter (J)
Recessive—the masked allele
a.
Nomenclature—lower case letter (j)
Genotype—an individual’s genetic makeup
a.
Reflects the allelic identity of that individual’s genes
b.
Character state of a gene defends on pattern of expression
Phenotype—which genes are expressed
a.
Refers to the expression of a specific trait
Hybrid—term describing offspring that receive alleles from two different sources
a.
Monohydrid involves a single trait
b.
Dihybrid involves two traits
Cross—refers to a mating event
a.
P refers to the parental generation
b.
F1 refers to offspring of a parental cross
i.
First filial generation
c.
F2 are offspring produced when F1 individuals are bred
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
II.
Sexual Sources of Genetic Variation
A.
1.
2.
3.
Three sources of variation
Independent assortment of chromosomes
Crossover of homologues
Random fertilization of eggs by sperm
B.
1.
2.
Segregation and independent assortment of chromosomes
Formation of tetrad during Prophase I of meiosis is random
a.
Maternal and paternal chromosomes are randomly distributed to daughter nuclei
b.
Occurs at meiosis I
Allele pairs are segregated during meiosis
a.
Distributed to different gametes
b.
A gamete has either the parental or maternal gene, but not both
i.
Some chromosomes may include elements of each due to synapsis
3.
Alleles on different pairs of homologous chromosomes are assorted independently of each
other
4.
Net result:
a.
Each gamete has a single allele for each trait
b.
Allele present is one of four possible parental alleles
5.
Number of different gametes resulting from independent assortment
a.
2n, where n is the number of homologous pairs
i.
Human’s: 223 or 8.5 million possibilities
C.
1.
2.
Crossing over (synapsis) of homologues and gene recombination (*See meiosis lecture)
Genes are arranged linearly along a chromosomes length
Genes on the same chromosomes are said to be linked
3.
a.
These genes are transmitted as a single unit during mitosis
During meiosis, paternal chromosomes can precisely exchange gene segments with their
homologous maternal counterparts
a.
This process gives rise to recombinant chromosomes
b.
Occurs at specific locations along the chromosome
i.
Chiasmata
D.
1.
Random fertilization
A single human egg will be fertilized by a single sperm
a.
Variation resulting from independent assortment and random fertilization: 8.5 million x
8.5 million or 72 trillion
III.
Types of Inheritance
A.
1.
Punnet square
“Bookkeeping” tool for determining possible gene combinations
a.
Only predicts probability of offspring with a particular genotype (and phenotype)
B.
1.
Types of inheritance
Dominant-recessive inheritance—interaction of dominant and recessive alleles where dominant
trait masks recessive trait
a.
Dominant traits are always expressed
i.
Any single dominant allele results in the expression of that trait
ii.
Homozygous or heterozygous
iii.
Phenotypes of homozygous dominant and heterozygous individuals are identical
b.
Recessive traits are only expressed as homozygous recessive state
2.
Incomplete dominance
a.
Character state of the heterozygote has a phenotype intermediate between homozygous
dominant and homozygous recessive
b.
Example: sickle cell amenia
i.
SS—normal RBC shape
ii.
ss—sickle shaped RBC due to substitution of one amino acid residue in ß chain
of hemoglobin
iii.
Ss—resistant to malaria; produces both normal and sickled RBC’s
3.
Multiple allele inheritance
a.
Some genes have more than two forms
i.
We only inherit two alleles for each gene
b.
ABO blood types
i.
ii.
IA, IB, i
IA and IB are co-dominant
Blood Group
O
A
B
AB
4.
Sex-linked inheritance—inherited genes on sex chromosomes
a.
b.
c.
d.
e.
5.
Genotype
ii
IA IA or IAi
IB IB or IBi
IA IB
X and Y are not homologous
Y chromosomes contains genes that determine maleness
i.
Y (15 genes) is 1/3 the size of X (2500 genes)
X codes for additional non-sexual characteristics
A gene found only on the X (and not Y) is said to be sex-linked
i.
Inheritance of sex-linked recessive genes cannot be masked by corresponding
gene on Y (i.e., there is no corresponding gene on the Y)
ii.
X-linked are never pasted from father to son (To be a son, must get Y from dad)
Characteristics present only on the Y are pasted onto male offspring and never to female
Polygenetic inheritance—different genes at different locations acting collectively
a.
b.
Qualitative phenotypes varying between two extremes
Skin color:
i.
Three separate genes
ii.
Two allelic forms per gene
iii.
A,a; B, b; C, c
iv.
A, B and C confer dark skin
v.
a, b and c confer pale skin
vi.
AABBCC genotype would be as dark-skinned as a human can be
vii.
aabbcc would be very fair
viii. Heterozygous condition at one or more alleles will result in a range of possible
pigmentations
IV.
Determining Genetic Makeup
A.
1.
2.
Laws of probability
Used to predict the probability of genetic events
Probability of independent events
a.
On event does not affect the occurrence of the other
b.
Probability of two events both occurring
i.
Individual probabilities are multiplied
c.
Probability of either event occurring
o.
Sum the individual probabilities
B.
1.
Test cross for heterozygosity
Cross an individual expressing the dominant phenotype and a homozygous recessive individual
a.
Dominant phenotype could be homozygous or heterozygous
i.
In complete dominance these genotypes express the same phenotype
Possible results
a.
If the dominant individual is heterozygous
i.
1:1 ratio of dominant to recessive offspring is observed
b.
If the dominant individual is homozygous
i.
All offspring express the dominant trait
2.
C.
1.
Dihybrid cross
Individuals different alleles at two loci
a.
Independent assortment when loci are on non-homologous chromosomes
b.
Phenotypic ratio of 9:3:3:1 is expected
V.
Inheritance and Chromosomes
A.
Linked genes do not assort independently
1.
Genes are located in a linear order on the chromosomes
2.
The principle of independent assortment does not exactly apply to loci on the same homologous
pair of chromosomes
3.
Linkage is the phenomenon where groups of genes are inherited together because they are
located on the same chromosome
4.
Crossing-over may skew the proportions of expected phenotypic outcomes
B.
The linear order of linked genes on a chromosome is determined by calculating the frequency
of crossing-over
1.
Segments of chromosomes are exchanged during crossing-over
a.
Percentage of crossing-over
i.
Sum the number of individuals in the recombinant classes
ii.
Divide by the total number of offspring
iii.
Multiply by 100
2.
Genes that are close together often are exchanged together
3.
Distance between two genes of a chromosome is measured in map units
a.
Map units measure the percentage of crossing-over
b.
One map unit represents 1% recombination
4.
Linked genes are composed of all of the genes on a particular chromosome
a.
Crossing over may affect whether all linked genes are inherited
B.
1.
2.
3.
Sex chromosomes
Determine the sex of the individual
a.
One sex will have a pair of identical sex chromosomes
i.
Human females have two X chromosomes
b.
The other sex has two different sex chromosomes
i.
Human males have one X and one Y chromosome
Y chromosome determines male sex in most species of mammals
a.
Contains a limited number of sex specific genes
i.
TDF—testicular defining factor
Anomalies
a.
XXY
i.
Klinefelter syndrome
ii.
Individual is male
b.
XO
i.
Turner syndrome
ii.
Individual is phenotypically female
C.
1.
Dosage compensation
Equalizes the expression of X-linked genes
a.
Typical females have two X chromosomes; males have one
b.
Inactivation of one X chromosome in female cells
i.
Barr body is a dark area of highly condensed chromatin
ii.
Single inactive X chromosome in female cells
VI.
A.
1.
Extensions of Mendelian Genetics
Dominance is not always complete
Codominance
a.
ABO blood group
B.
1.
A single gene may affect multiple aspects of the phenotype
The ability of a single gene to have multiple effects is referred to as pleiotropy
D.
1.
2.
Alleles of different loci may interact to produce a phenotype
These interactions result in variations from typical expected Mendelian ratios of crosses
Epistasis is the interaction between genes such that one gene can influence the effect of another
gene
E.
1.
2.
3.
4.
5.
Polygenes act additively to produce a phenotype
Multiple, separate genes have similar and additive effects on the morphological feature
Height and skin color are simple examples in humans
F1 generation has phenotypes intermediate between the homozygous parents
F2 generation shows wide variation in phenotypes
Bell-shaped curves (a normal distribution) are indicative of polygenes
F.
1.
2.
Genes interact with the environment to shape phenotype
Human height is polygenic and also environmentally influenced
The range of phenotypic possibilities that can occur under different environmental conditions is
the norm of reaction
This brings to mind the idea of nature versus nurture
3.