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Inheritance

Back in the day, Mendel crossed different varieties of pea plants to establish rules of inheritance. He didn't know what we know today about genes and DNA. So what do we know, and what did he find out?

The entirety of genetic material in an organism is called a genotype. It can also refer to specific things, like a genotype for a certain trait in a given organism.

The genotype refers to the physical constitution of a little part of DNA. Its expression, however (that is what protein a gene encodes, and what that protein ends up doing in the organism) is a separate entity which is subject to environmental influence. This is called the phenotype.

Humans have 2 sets of chromosomes, so for each distinct chromosome e.g. chromosome 1, there are two copies. How do the same genes on both homologous chromosomes interact if they result in different phenotypes? Which has priority?

If you crossed a green pea plant with a yellow pea plant, what colour would the offspring be? What about their offspring's offspring?

This is precisely what you'll be able to answer by thee end of this topic.


Basics

Versions of the same gene that give rise to different phenotypes are called alleles. For example, the gene responsible for ear shape in a cat may have 2 alleles: pointy shape and oval shape.

One of these alleles may be expressed at the expense of another, where both are present together in a cat. Say that the oval shape allele is dominant while the pointy shape is recessive.

Because cats also have 2 copies of each chromosome (and therefore gene), these alleles are written as 2 letters. 

If the gene for ear shape is abbreviated as E or e for ear, then the alleles would be:

E (dominant) for oval shape and 
e (recessive) for pointy shape

These upper case - dominant, lower case - recessive notation rules are used universally. So what combinations may be present in a cat?

EE, ee or Ee

The first two examples are called homozygous because the same allele (E or e) is present twice, while the last example is heterozygous because different alleles (E and e) are present.

So what happens in crossing?

EE x ee gives rise to 4 combinations: Ee, Ee, Ee and Ee! 100% heterozygous where the cats will appear oval-ear shaped, yet also carry the recessive allele for pointy ears. Let's do a second cross.

Ee x Ee gives rise to 4 combinations: EE, Ee, eE and ee. That's 50% homozygous and 50% heterozygous. 3/4 will have oval ears while 1/4 will have pointy ears.





Now see what happened in Mendel's pea plant experiments?

It's also possible to have multiple (more than 2) alleles for a given gene. Say there is also a round ear allele. It could be that this allele is codominant with the pointy ear allele, so that both traits are simultaneously expressed.

These cases represent monohybrid inheritance. Regardless of how many alleles there are for one gene, we are only looking at one gene and its associated trait - hence monohybrid.




Dihybrid inheritance refers to the inheritance of two genes, each with multiple alleles. Increasing the number of compared genes therefore produces a greater variety of genotypes in the offspring.

This showcases Mendel's first law that states factors are segregated. Remember, in that time DNA and the concept of a gene weren't known. Segregation of factors referred to the factors (genes) being inherited independently from each other. The second law refers to the independent assortment of these factors during inheritance. This resonates in particular to the process of meiosis where chromosomes are assorted into offspring cells independently of each other. That is, small heritable units e.g. genes are subject to their own heritability, and individuals do not simply inherit their parents' genetic information in bulk.

It is possible that some allelic combination are lethal and do not result in viable offspring.




If a dominant individual is not known to be homozygous or heterozygous, a test cross can be done where the unknown individual is crossed with a recessive (and hence homozygous) individual. The phenotype proportion of the offspring can reveal whether the dominant parent is homozygous or heterozygous.

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