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After MendelCh. 8 – 3 (con’t)
Beyond Dominant And Recessive Alleles
Dominant/recessive inheritance is the simplest type of gene interaction
What causes dominance?
The dominant allele codes for a polypeptide that
WORKS!
The recessive allele doesn’t
Pedigree – a diagram that shows the occurance of a genetic trait in several generations of a family
Autosomal or Sex-linked
Autosomes – any chromosome that is NOT a sex chromosomeIn humans, 22 pairs of autosomes (44 chromosomes)
Sex chromosomes – chromosome pair that is different in males and females X chromosome - sex chromosome; in humans and many other organisms females have 2 X’s and males have only 1 Y chromosome - male sex chromosome (in humans and many other organisms)
ex. In humans XX = female XY = male
Sex Determination
XX x XY
Genes on the Sex Chromosomes
In addition to determining the sex of an individual, sex chromosomes carry genes that affect other traits (not related to sex)
Sex-linked - a trait that is determined by a gene found on one of the sex chromosomes, such as the X (and Y) chromosomes in humans (most are X-linked)
Thomas Hunt Morgan (early 1900’s) discovered the 1st sex-linked trait in Drosophila Studied Drosophila melanogaster, fruit fly
Found: that Mendel’s principles applied to other organisms
- Small and easily raised and kept
-Has obvious characteristics and easy to distinguish male and female -Has a short life span
-Produces a large number of offspring
-Has only 4 pairs of chromosomes, which are very large
The Characteristics of Drosophila
Morgan’s Research
Bred and looked at many fruit flies looking for an interesting traitFound a white-eyed male
Crossed:
P gen: normal red-eyed female x white-eyed male
F1 gen: all offspring red-eyed (allowed to interbreed)
F2 gen: got expected 3 red-eyed : 1 white-eyedBUT all white-eyed were MALES
P gen
F1 gen All F1 offspring are red-eyed
Morgan hypothesized that gene for eye color in Drosophila was sex-linked and he performed a test cross.
Test cross 1Crossed red-eyed female (F1) x white-eyed male
Test cross 2Crossed red-eyed male (F1) x white-eyed female
Morgan concluded: that the gene for eye color (in Drosophila) is carried on the X chromosome and there is NO corresponding allele on the Y chromosome(Y is smaller so it is reasonable to assume that the Y is missing genes)
carrier – heterozygous (female) for a (sex-linked) trait
Sex-linked (X-linked) traits in humans red-green colorblindness
hemophiliaDuchenne muscular dystrophyALD (adrenoleukodystrophy)
Chapter 8 - 4
Complex Patterns of Heredity (pg. 177)
Since Mendel, scientists have discovered many genes that do NOT follow the simple patterns of inheritance described by Mendel (he described dominant/recessive inheritance).
Many time genes exhibit more complex patterns of inheritance (or gene interaction)
Traits Influenced by Several Genes
Polygenic Trait - trait controlled by 2 or more gene pairs; a characteristic of an organism that is determined by many genes
Ex.
All of these traits have degrees of intermediate conditions between one extreme and the other
Example: Trait controlled by 3 gene pairs
Characterized by: F1 generation with phenotype intermediate to parent phenotypes F2 generation with a phenotype ratio of 1: 2: 1
(instead of Mendel’s 3:1
Incomplete Dominance -
Condition in which a trait in anoffspring is intermediate betweenphenotype of the 2 parents because the dominant allele is unable to express itself fully;
“blending” inheritance
Incomplete Dominance
Traits with Two Forms Displayed at the Same Time
Codominance - Condition in which BOTH alleles of a gene are fully expressed
Both alleles are active (or expressed)
Example: coat color in horses (or cattle) Roan = mixture of red and white hairs
Codominance
P gen Red coated x white coated
CRCR x CWCW
F1 gen All roan coated
All CRCW
F2 gen
1 Red : 2 Roan: 1 White
1 CRCR : 2CRCW; 1CWCW
P generation F1 Cross
F2 Offspring
Traits Controlled by Genes with Three or More Alleles
Multiple Alleles – 3 or more alleles for a genetic trait
Individual still has only 2 alleles (1 from each parent)
Ex. Human ABO blood groups3 Possible Alleles: IA, IB, i
Gene codes for an antigen (protein) on red blood cells
4 Phenotypes 6 Genotypes
Type A IA IA or IA i
Type B IB IB or IB i
Type AB IAIB
Type O ii
Gene Interaction
A and B are codominant
A and B are dominant over O
O is recessive
Human Blood Typing
Transfusion possibilities
Type (antigen)
Antibodiesin plasma
Can Receive From
Can Donate To
A Anti-B antibodies
A, O A, AB
B Anti-A antibodies
B, O B, AB
AB Neither A, B, AB, O AB
O Anti-A antibodies
Anti-B antibodies
O A, B, AB, O
Blood Typing
Anti-A antibodies
Anti-B antibodies
Multiple Alleles – Coat Color in Rabbits
Full ColorC alleleCC or Ccchd or Cch or Cc
Chinchilla (dark) cchdcchd or cchdch or cchdc
Chinchilla (light) or SableCchlcchl or cchlch or cchlc
Himalayan chch or chc
Albino cc
Traits Influenced by the Environment
Hydrangea flower color is determined by soil pHAcidic soil results in blue flowersNeutral to basic soil results in pink flowers
An individual’s phenotype may depend on conditions in the environment - some genes are influenced by environmental factors, some are NOT
Acidic soil Basic soil
Arctic Fox - fur color affected by temperature
In summer with warm temperaturesenzymes are produced that make pigment thatdarkens fur
In winterWith cold temperaturesenzymes needed for pigment production do NOT function resulting in white fur
Fur color in Siamese cats
Influenced by temperature
Dark fur produced at body locations that are cooler than normal body temperature(extremities - nose, ears, paws and tail)
Epistasis – a phenomenon in which one gene alters the expression of another gene that is independently inherited
B = Black
b = brown
C = controls whether any pigment is deposited in the hair
c = no pigment in hair
All offspring with cc are white regardless of the black/browngene
Siamese cats. One gene controls the color of the pigment, and black hair (B) is dominant to brown hair (b). The other gene controls the dilution of the pigment in the hairs, with dense pigment (D) being dominant to dilute pigment (d).
Genotypes
PhenotypeF2
ratio
BBDD BBDdBbDD BbDd
“seal” (black dense) 9
BBdd, Bbdd
“blue” (black dilute) 3
bbDD, bbDd
“chocolate” (brown dense)
3
bbdd “lilac” (brown dilute) 1
BB = black Lab, no chocolate gene Bb = black Lab,
bb = chocolate Lab, no black gene
Yellow is produced by the presence of a recessive epistatic gene which has the effect of blotting out the expression of the black or chocolate genes.
If "E" = the dominant form of for the epistatic gene, and "e" = the recessive form, then there are three possibilities:
EE = no yellow gene Ee = yellow carrier but apears either black or chocolate
ee = yellow Lab
= or or or
= or or
= or