Genetics After Mendel     

Types of Dominance1. Complete

-1 gene masks the other (Tt…see Mendel)

2. Incomplete -Neither gene is dominant, neither is recessive.-The gene effect is mixed-We use 2 different letters for the

homologous pair since neither is dominant.

Use a Punnet square to predict offspring!

RW RW

RW RW

R R

W

W

100% PINK!!!

Red Flower – RR White Flower - WW

Try this one:• Bird feathers show incomplete dominance. For

example, the gene for black feathers, B, may be incompletely dominant to the gene for white feathers, b. The heterozygous condition produces a bird with blue feathers. Determine the genotypic and phenotypic ratios that will result from each of the following crosses:

A) blue x whiteB) black x blueC) blue x blue

Answers:A) 50% Bb (Blue); 50% bb (White)

B) 50% BB (Black); 50% Bb (Blue)

C) 25% BB (Black); 50% Bb (Blue); 25% bb (White)

Types of Dominance3. Co-Dominance

-Neither gene is dominant or recessive-Gene effects are not mixed-Both genes are expressed at the same

time!-Eg. Roan Cattle – Red hairs and white hairs

THERE’S NO NEED TO MOAN ABOUT

BEIN’ ROAN!

Co-Dominance Cont.-Write C (for co-dominance) and a super script (R=Red, W=white)-Red haired = CRCR

-White haired = CWCW

-Red and White (Roan) =CRCW

Use a Punnett square to predict offspring:

1) Cross a white cow with a red bull

X

CRCW CRCW

CRCW CRCW

CR CR

CW

CW

2) Cross a white cow with a roan bull

100% Roan!!

A Special Case – Blood Type

• There are three alleles! A B O• A and B are co-dominant• O is recessive• FYI: +/- is the Rh factor (If the

major Rh antigen is present, the person is Rh+; if not, they are Rh-)

Predict offspring bloodtypes for: a) Heterozygous type A mother, type

O father

Blood Type… Cont.

AO OO

AO OO

A OO

O

b) Mother: AB; Father: Hetero type B

50% AO50% OO

Try these ones:•Mother: AB; Father: Hetero type B•Match the children and parents:

Parent Set ChildrenO X O ABAB X O BA X O AAB X A O

What’s your type?•AB universal plasma donors

•AB+ universal recipients

•O+ blood most needed

•O- universal RBC donor

    TYPES     DISTRIBUTION RATIOS

      O + 1 person in 3 38.4%

      O - 1 person in 15 7.7%

      A + 1 person in 3 32.3%

      A - 1 person in 16 6.5%

      B + 1 person in 12 9.4%

      B - 1 person in 67 1.7%

     AB + 1 person in 29 3.2%

     AB - 1 person in 167 0.7% 

X-Linked Traits

•All genes found on the ‘X’ chromosome are “linked”.•Many of the genes on the ‘X’ are not on the ‘Y’.•The ‘X’ is larger and has many genes that are not related to gender… (i.e. colour vision, blood clotting etc.)

Are you colour blind?

WHAT DO YOU SEE?

X-Linked Traits•If a recessive gene is on the ‘X’:- always expressed in males - no homologous pair on the ‘Y’.- recessive, X-linked diseases are much more common in males!

Hemophilia – I can’t stop bleeding!!

Since gender and the gene are important, both must be noted.

•Dominant allele – H•Recessive allele – h•XH – dominant gene on ‘X’•Xh – recessive gene on ‘X’•Y – no gene on the ‘Y’

Hemophilia

• Female genotypesXHXH – NormalXHXh – Normal but a ‘Carrier’

XhXh – Hemophiliac (very rare)• Male genotypes XHY – Normal XhY - HemophiliacPredict the offspring for: -Mother carrier and a normal father!