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