Genotypes and Phenotypes

What can allele be

Genes have different forms called alleles. An allele can be recessive or dominant.

What happens when a dominant gene is paired with a recessive gene

The dominant allele determines the characteristic

How can a recessive gene produce a recessive phenotype

the individual must have two copies, one from each parent

What is a genotype

The genotype is the set of alleles of a gene in our DNA responsible for a particular trait.

What is a phenotype

The phenotype is the physical expression, or characteristics, of that trait.

Genotype and phenotype relationship

A phenotype is how the genotype is expressed.

Homozygous

when both alleles are the same

Heterozygous

when alleles are different

AA

Homozygous dominant

Aa

Heterozygous

aa

homozygous recessive

Hemizygous

only one copy of gene instead of two. All the genes on the single X chromosome in males are hemizygous

Monohybrid cross

Mating between two individuals, investigating one gene

Male and female sex chromsomes

Male; XY

Female; XX

However, there are variations XXY (Klinefelter syndrome) or XO (Turner syndrome).

Sex linked

Why do X-linked recessive conditions occur more often in males?

Recessive trait cannot be hidden since there is no second allele

Whereas females with their second X get a second chance.

ONLY females can be carriers.

Sex linked conditions dominant/recessive

It must be carried on either an X or Y chromosome.

Typically, there are more X linked conditions

since the x chromosome it much bigger than the Y chromosome and therefore can carry more genes.

What are the mendal laws

Law of dominance

Law of segregation

Law of Independent assortment

What is the law of dominance?

In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

What is the Law of segregation?

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

What is the Law of Independent assortment

Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

What traits don't follow the dominant/recessive rules of Mendels

Codominance

Incomplete dominance

Multiple Alleles

Lethal alleles

Multiple Genes (polygenic)

Environmental Factors (epigenetic)

What is incomplete dominance

Ratio will not fit the expected dominant / recessive pattern

What happens in incomplete dominance?

A heterozygous genotype will produce a mixed phenotype.

This new, third phenotype is a blend of both parents, cannot clearly see either parental phenotype.

Since neither phenotype completely hides the other, there is no dominant or recessive allele.

Codominance meaning

Heterozygote produces a third phenotype. Both parental phenotypes are clearly seen.

Multiple Alleles meaning

It is possible that a trait has more than two (multiple) alleles that determine which phenotype is expressed.

Ex.

hair colour in rabbits

blood groups in

humans.

Multiple alleles and Codominance in ABO Blood Group

The ABO blood group system is the classification of human blood based on the inherited properties of red blood cells depending on the antigens A and B, which are carried on the surface of the red cells.

People may have type A, type B, type O, or type AB blood.