4.3 Theoretical genetics

Definitions
Genotype - allele combination of an organism.

Phenotype - characteristics of an organism.

Dominant allele - an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.

Recessive allele - an allele that has an effect on the phenotype when it is present in homozygous state.

Co-dominant alleles - pairs of alleles that both affect the phenotype when in a heterozygote.

Locus - particular position on homologous chromosome of a gene.

Homozygous - two identical alleles of a gene.

Heterozygous - two different alleles of a gene.

Carrier - an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for the allele.

Test cross - testing suspected heterozygote by crossing with known homozygous recessive.

Monohybrid Cross with Punnett grid

Monohybrid Cross (rosyboas.to)

 

Multiple alleles

Some genes have more than two alleles (multiple alleles).

Codominance (expressed equally in phenotype).

Incomplete dominant (neither fully expressed, blending).

Dominance order (Allele A>B>C).

ABO blood groups

Genotype I^AI^A gives blood group A/I^BI^B gives group B

Neither I^A or I^B are dominant over each other, another blood group is formed called IAB

Genotype I^Ai and I^Bi give blood groups A and B

 

Blood Types (ib.bioninja.com.au)

 

Blood type A contains:

A antigen.

Anti B antibody.

Therefore, cannot have B or AB blood, only A and O blood.

Blood type B contains:

B antigen.

Anti A antibody.

Therefore, cannot have A or AB blood, only B and O blood.

Blood type AB contains:

A and B antigens.

Neither A nor B antibodies.

Therefore, this type is the universal recipient.

Blood type O contains:

Neither A nor B antigens

Both antibodies

Therefore, this type is the universal donor and can only have O blood

How sex chromosomes control gender

X chromosome is large and contains many genes essential to both male and female development. Y chromosome is smaller and has fewer genes. One gene, called TDF, is found on the Y gene only, and is needed for testis and testosterone development. Females have 2 X chromosomes and ovaries are developed and female sex hormones are produced. All of us get mother’s X and daughters get father’s X while sons get father’s Y.

X and Y chromosomes

Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

Sex linkage definition

Sex linkage - gene controlling a characteristic is found on a sex chromosome (associate trait with predominant gender)

Inheritance of colour blindness and hemophilia

Females with normal color vision can be heterozygous, possessing the dominant allele and recessive allele (that causes color blindness). In this case, she is a carrier, but shows no signs of the blindness due to the dominant allele masking her recessive one. She can also either be homozygous dominant and again, she has normal color vision. It is very rare for females to be homozygous recessive for color blindness, there is only a 25% chance of happening.

For males, genes are found on the X and not the Y. So, we have a 50% chance of getting color blindness. Even if we have a recessive allele for blindness, the Y cannot mask this allele because it is insignificant. Males will receive this condition if the X-linked recessive allele is passed from mother since males always get the father’s Y.

Same case with hemophilia. For color blindness, the notation is X^B  and for hemophilia it is X^H. For recessive, make the B and H not capitalized.

Sex-linked genes in females

Human female can be homozygous or heterozygous with respect to sex-linked genes.

Female carriers of X-linked recessive alleles

Individual with recessive allele for disease masked by condition of dominant allele, which means this individual is a carrier. This is heterozygous, however, it can be passed onto next generation and they will not suffer from defect.

Females can be carriers, 2 X chromosomes, however, males cannot because the Y does not play a role and the genes are found only on the X. So if males receive the defect from the mother, the male will suffer from the defect.

Predict genotypic and phenotypic ratios of offspring of monohybrid crosses

X-Linked Punnett Grid (merckmanuals.com)

Blood Codominance Punnett Grid (fhs-bio-wiki.pbworks.com)

Pedigree Charts

In the case of a characteristic due to a dominant allele, the characteristic tends to occur in one or more members of the family in every generation.

A recessive characteristic is seen infrequently, often skipping many generations.

Square for male, circle for female.

If square or circle is colored, means they are affected with a disease; if dotted, means they are carriers.

For example Albino people have homozygous recessive allele (pp) while normal have homozygous normal (PP) or carriers (Pp).

If most of the males in the pedigree are affected the disorder is X-linked.

If it s a 50/50 ratio between men and women the disorder is autosomal.

If the disorder is dominant, one of the parents must have the disorder.

If the disorder is recessive than neither of the parents has to have the disorder as they can be heterozygous.