SCIPAD: Types of monohybrid inheritance

How do you know when you are dealing with a question involving co-dominance?

when offspring shows a third phenotype which contains characteristics of both the parental phenotypes

How do you know when you are dealing with a question involving incomplete dominance?

when the offspring shows a third phenotype which is a blend of the parental phenotypes

Complete dominance

where the dominant allele completely masks the effect of the recessive allele in heterozygous conditions (i.e. when describing pea flower colour, an individual with the genotype Pp would show a purple flower phenotype. The dominant 'P' allele completely masked the recessive 'p' [white] allele.)

XY

Male

True or false: "If you can simplify a genotypic or phenotypic ratio, then you must always do so."

True

XX

Female

Lethal Alleles (page 138)

- alleles that produce a phenotypic effect that causes the death of the organism - usually arise due to a mutation in an essential gene - mutation means the gene cannot produce a functioning version of an essential protein - may be completely dominant, incompletely dominant, or recessive

Multiple Alleles (page 136)

- genes can have more than two different alleles - genes for human blood groups have multiple alleles - some of these alleles show complete dominance, other shows co-dominance - Human blood groups (ABO blood groups) controlled by three alleles - (1) IA codes for A-type blood (2) IB codes for B-type blood (3) i codes for O-type blood - IA and IB are co-dominant, both dominate i completely

Multiple alleles

About 30% of the genes in humans are di-allelic, that is they exist in two forms, (they have two alleles) About 70% are mono-allelic, they only exist in one form and they show no variation A very few are poly-allelic having more than two forms (multiple alleles)

Lethal alleles

If an allele fails to code for an essential protein it is a lethal allele. Most lethal alleles are recessive. Recessive lethal alleles (usually) cause death in the homozygous form. Dominant lethal alleles are possible but are quickly eliminated as it usually causes death before the individual reproduces. Occasionally in lethal alleles one copy of a normal allele does not produce enough protein so the heterozygote (carrier) has a different phenotype from either homozygote.

Co-dominance

In the case of 'co-dominance' both alleles are expressed equally

Incomplete dominance

In the case of 'incomplete dominance,' neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes. The heterozygous individuals are a combination of both the dominant and recessive conditions.

Co-dominance (page 135)

Occurs when a single gene has more than one dominant allele. An individual who is heterozygous for two co-dominant alleles will express the phenotypes associated with both alleles. (i.e. a pure breeding red flower (RR) is crossed with a pure breeding white flower (WW), and the offspring have both red and white petals (RW)). Both alleles are seen in the phenotype, so the alleles are co-dominant.

To work out the genotypic ratio,

count the number of times that each genotype is shown within the square, then separate the numbers with a colon.

Heterozygous

individual is producing both proteins that contribute to the final phenotype

We can also use Punnett squares to work out phenotypic ratios. To do this, we

simply replace the offspring genotypes with the corresponding phenotypes. Remember, being tall (T) is the dominant trait, and short (t) is recessive.

Remember that if dominant alleles are present, they will

they will always be expressed.

Having two heterozygous parents results in

three different genotypes, GG, Gg and gg.