How are characteristics (or traits) passed from parents to offspring?
In this section we examine some of the basic mechanisms of Mendelian genetics
and related aspects of inheritance.
What is Mendelian genetics?
At its most basic, Mendelian genetics examines how traits with 1 gene
and two alleles are passed on from parents to offspring. By understanding
Mendelian genetics, you will understand how traits can "skip" a generation,
why you get a 3:1 phenotypic ratio following a cross between two heterozygotes
(or an F1 x F1 cross who's parents were homozygous dominant and
homozygous recessive), and may more aspects of inheritance.
Some vocabulary to understand:
Phenotype (Phenotypic ratio)
Genotype (Genotypic ratio)
F1 & F2 generations
Law of Segregation
Although very simple, understanding this law is essential to understand
both phenotypic and genotypic ratios, and therefore the chance (= the
probability) an offspring will have a particular genotype and phenotype.
This law states that gametes have an equal chance of carrying either of
the two alleles carried by the parent for each gene. Thus 50% of gametes
will carry one allele and 50% the other. Clearly, where a parent is homozygous
for a gene, 100% of the gametes will carry the same allele.
Non-Mendelian mechanisms of inheritance (other mechanisms of inheritance)
Incomplete or codominance
Traits with one gene with multiple alleles or multiple genes each with
2 or more alleles
What determines a phenotype?
3) Genotype x Environment interaction
What causes variation?
Let's focus ONLY on genetic causes of variation
Allele Combination: The particular combination of alleles for all genes
you have will influence your phenotype. There are an astronomical number
of possible allele combinations. If, for example, we assume that each
gene only has two alleles (we know this not to be so), the number of possible
allele combinations can be calculated as follows: 2n where
n = number of genes.
Mutation is the ultimate source of new alleles.
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