3.4: Inheritance

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Describe conclusions drawn from Mendel's pea plant experiments.

Understanding: Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

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Through selective breeding of pea plants, Mendel discovered that certain traits show up in offspring without blending of the parent's characteristics. Mendel observed seven traits: flower color, stem length, seed color, pod color, flower position, seed shape and pod shape.

Mendel concluded:
1. genetic "units" of inheritance are passed from parents to offspring

2. the offspring inherits one "unit" from each parent for each trait.

3. the "unit" may be masked or hidden (i.e. recessive) in an individual but can still be passed on to the next generation.

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Many genes have multiple alleles within the population. For example, in ABO blood typing there are three common alleles for the Isoagglutinogen gene: I^A, I^B and i.

Within a diploid individual there may only be a combination of two of the alleles:
I^A, I^A (type A)
I^A, I^B (type AB)
I^A, i (type A)
I^B, I^B (type B)
I^B, i (type B)
i, i (type O)

State the maximum number of alleles in a diploid zygote.

Understanding: Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

Alleles are variations of a single gene. Although there usually are multiple alleles for a gene in the population, any single individual can only have a maximum of two alleles of a gene, one allele on each chromosome of a homologous pair.

Define "dominant allele."

Understanding: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

Different versions of a gene are called alleles. Dominant alleles show their effect even if the individual is heterozygous, they can mask the presence of another allele.

Define "recessive allele."

Understanding: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

Different versions of a gene are called alleles. Recessive alleles only show their effect if the individual has two copies (homozygous recessive), otherwise their presence can be masked by a dominant allele.

State the usual cause of one allele being dominant over another.

Understanding: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

The cause of allele dominance is complex and can vary between genes. However, in general, the dominant allele codes for a functioning proteins whereas the recessive allele codes for a less (or non-) functioning protein.

Sometimes the recessive allele is the "normal" or "healthy" version of the gene.

Define "codominant alleles."

Understanding: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

With codominant alleles, both alleles are expressed equally; there isn't masking of a recessive by a dominant allele.

Using the correct notation, outline an example of codominant alleles.

Understanding: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

Since there isn't a true dominant allele, a lowercase letter is NOT used when alleles are codominant. Rather, two different capital letters are used and places as superscript next to a common letter that represents the name of the gene. For example, type A and type B alleles of the Isoagglutinogen gene.

I^A and I^B are codominant.

Define "carrier" as related to genetic diseases.

Understanding: Many genetic diseases in human are due to recessive alleles of autosomal genes.

A genetic carrier is an individual that has inherited a recessive allele of a gene but does not display the symptoms of the disease because they also have the dominant (normal functioning) allele. Carriers are heterozygous.

3.4: Inheritance

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