Science 103 Spring 2006 – Outline 16
Reading:
Multiple alleles, autosomes & sex chromosomes, codominance, & sex linkage
P 563 and 566
Mendelian Genetics Problems
1. Monohybrid Cross
The presence of freckles on human skin is a dominant trait controlled by a single
gene pair.
(a) What is/are the possible genotype(s) of an individual with freckles?
(b) What is the probability that 2 parents without freckles will produce a child with
freckles?
(c) If 2 parents with freckles produce a child without freckles what is the probability that
their next child will:
(i) also be without freckles?
(ii) have freckles?
2. Dihybrid Cross
You have performed Mendel’s dihybrid cross using the traits for seed shape and
seed color. In the F2 generation you obtain plants with round, yellow seeds. You decide
to determine the genotype of ONE of these plants. In your cross, you obtain progeny
with the following phenotypes. 25% produce round yellow seeds, 25% produce round
green seeds, 25% produce wrinkled yellow seeds, and 25% produce wrinkled green
seeds.
(a) Describe the cross you performed to determine genotype.
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(b) What is the genotype of the F2 plant you tested?
Non-Mendelian Patterns of Inheritance
1. Mendel’s Characters
Mendel chose characters that have a simple genetic basis. Each character is
(a) controlled by only 1 gene, for which there are only 2 alleles.
(b) One allele is completely dominant to the other.
2. Non-Mendelian Traits
The conditions above are not met by all heritable characters.
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Multiple Alleles
A gene can have >2 alleles within a population.
Codominance
(In Mendelian genetics 1 allele is completely dominant over the other).
1. Description
2. Example of Multiple Alleles and Codominance – ABO Blood type
(a) Gene Function:
(b) Immunity:
(c) Alleles:
There are 3 different alleles for gene I:
(a) IA and IB
I=
(b) i
(d) Phenotypes:
There are 4 phenotypes –
(i) IAIB Heterozygotes
(ii) IAi and IBi Heterozygotes
 IAi 
IBi 
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(iii) Correlation between Genotype and Phenotype
Phenotype
A
B
AB
O
Genotype
(e) Immune Response
An individual’s immune system will reject cells coated with a sugar it does not
recognize as “self” (important for blood transfusions). (Has Ab to non-self).
Key
+ = Accept
- = Reject
Recipient\Donor
A
B
AB
O
A
B
AB
O
3. Example of Multiple Alleles – Rhesus Blood Group
(a) Multiple Alleles for this Trait
(Usually treated as a 2 allele system, but is actually much more complicated).
(i) Number of Alleles
(ii) Phenotype
(b) Dominance and Frequency
(i)
(ii) ~85% of the population is Rh+.
(c) Anti Rh Antibodies
Eg.
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(d) Erythroblastosis Fetalis
(Hemolytic disease of newborns)
(i)
(ii)
(iii) When does the Problem Occur?
(iv) Prevention
Autosomes and Sex Chromosomes
1. Autosomes
(a) Description
(b) Number
2. Sex Chromosomes
(a) Principle Function
(b) Sex Differences
(i) Female Humans –
(ii) Male Humans –
3. Sex Determination
Phenotype
Genotype
Female
Male
Gametes
Punnett Square:
Gametes
X
X
X
Y
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F1 Progeny:
4. Features of the Y Chromosome
5. Sex Linked Genes
Example of a Sex-Linked Recessively Inherited Disease – Royal Hemophilia
1. Description of Hemophilia
2. Cause
3. Sex-Linked Hemophilia
(Not all hemophilias are sex-linked, most are autosomal, but all are recessive).
(a) Description
Carried on the X chromosome, therefore any male with the recessive allele (XhY)
will have the disease and any heterozygous female (XHXh) will be a carrier.
(b) Royal Hemophilia
Queen Victoria was a carrier.

Pedigrees
Family trees showing which individuals carry a particular trait.

Pedigree Symbols
= Male
= Female
Cross =
Affected Individuals =

Royal Pedigree for Hemophilia
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