Download Inheritance Patterns Simple dominance, incomplete dominance

Inheritance Patterns
Simple dominance, incomplete dominance, codominance, sex-linkage polygenic inheritance
Applying Mendel’s Laws
 We can use Mendel’s Law of Segregation to predict the outcome of possible crosses of
individuals
 First, we need some terminology
o Genotype: the alleles an organism has (e.g. I have the genotype bb because I have two
copies of the blue eyes allele)
o Phenotype: the characteristics an organism displays (e.g. my phenotype is blue eyes)
o Homozygous – a genotype that has two identical alleles (bb is a homozygous genotype
because both alleles are the same)
o Heterozygous – a genotype that has two different alleles (Bb is a heterozygous
genotype because the alleles are different)
Mendel & Inheritance Patterns
 Mendel observed only a particular type of inheritance pattern
 His pea plants had an inheritance pattern of simple dominance.
 Type of alleles in simple dominance:
o Dominant allele: always appears in the phenotype, whether it is part of a homozygous
or heterozygous genotype
 e.g. if a person is heterozygous for polydactyly (having more than 5 fingers) – Pp
– they will have an extra finger
o Recessive allele: two copies of the allele must appear in the genotype (it must be
homozygous) for the trait to appear in the phenotype
 tongue rolling – you will only not be able to roll your tongue if you have genotype
rr
Simple Dominance: Using the terms
 We use capital letters for dominant alleles
 We use lower case letters for recessive alleles
 The letters for the alleles should be the same (e.g. F for purple flowers allele, f for white
flowers allele)
Monohybrid crosses
 If we are looking at the inheritance of only one trait,
we are looking at a monohybrid cross
 We can predict the outcome of a monohybrid cross
when there is simple dominant-recessive
inheritance
 A Punnett grid helps us to do this.
 We know from Mendel’s Law of Segregation, that
gametes contain only one allele for each trait
 So, if a purple pea plant is heterozygous for flower
color, its gametes will either have a F allele
(purple) or a f allele (white)
 A homozygous white pea plant’s gametes can only
have a f allele (white)
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We can show this in a Punnett grid
When we fill in the grid, the center boxes show us the possible
genotypes of the offspring
o The genotypic ratio for the outcome of this cross is 1:1, Ff:ff
o The phenotypic ratio is also 1:1, purple:white
Incomplete Dominance
 For some genes, neither gene is fully dominant over another.
 In incomplete dominance, the F1 phenotype will be somewhere in
between the two parental phenotypes.
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In this type of inheritance pattern, the
heterozygote produces less red pigment, so the
F1 generation flowers are pink.
If the F1 offspring are crossed, what would you
predict the outcome would be?
Codominance
 Codominance is similar to incomplete dominance.
 In a heterozygous individual, however, each of the
alleles will produce their own distinct product.
 Example: Blood type
 Blood types differ due to proteins that are present on
the outside of red blood cells.
 In a person who is a heterozygote for the A and B
alleles, both the protein for type A and the protein for
type B will be produced.
 Predict what the possible outcomes are for a child
who has one parent with type A blood and one
parent with type AB blood.
Sex-linkage
 When genes are carried on the sex chromosomes, unique
patterns of inheritance are created.
 Example: Hemophilia is a recessive trait carried on the X
chromosome.
 Predict the outcome of a cross between a normal man and his
wife, who is a carrier (heterozygous) for hemophilia
Dihybrid Crosses
 If we want to predict the outcome of a cross
involving two genes, we use a dihybrid cross.
 Example: round yellow seeds x green
wrinkled seeds.
Polygenic Inheritance
 Traits aren’t always controlled by only one gene.
 Polygenic Inheritance happens when more than one
gene affect a single phenotypic characteristic.
 Example: Skin color in humans is controlled by at least
three different genes (and probably more)
Your Task:
 Follow the link from Edmodo to the Mendelian Genetics problem sets from the Biology Project
 Do the monohybrid crosses, dihybrid crosses, and the first sex-linked inheritance problem set.
 Use the tutorials to help you understand problems with which you have difficulty.