In this article, we are going learn about genetic inheritance 

Terminology You Should Know for Genetic Inheritance

• Genome: Entire DNA of an organism
• Gene: A section of a DNA that codes for a specific protein, thereby contributing to a certain characteristic (E.g. gene that determines eye colour, gene that codes for Huntington's disease)
• Chromosome: A structure found in the nucleus that is made up of a long strand of DNA. Genes are located on chromosomes
• Homologous chromosome: In a pair of homologous chromosomes, one chromosome is inherited from the maternal parent, and the other chromosome is inherited from the paternal parent.
• Allele: Different versions of a gene. 
  • For example, a gene coding for eye colour can have two different versions: an allele for blue eye colour and another allele for brown eye colour
  • Each homologous chromosome contains the same gene, but the pair of homologous chromosomes may have different alleles.
  • Organisms have a pair of alleles: one allele comes from the father, and the other allele comes from the mother.
• Dominant and recessive allele
  • A dominant allele is expressed even when it is paired with a recessive allele. A dominant allele is usually expressed as uppercase letters.
  • A recessive allele is expressed only when there is no dominant allele present and is paired with another recessive allele. A recessive allele is usually expressed as smallcase letters.
• Homozygous and heterozygous
  • Homozygous means that the same allele is present on both homologous chromosomes.
  • Heterozygous means that different alleles are present on homologous chromosomes.
• Genotype and phenotype
  • Genotype is the genetic makeup of a particular trait.
  • Phenotype is the observable characteristic of an organism that results from its genotype.

You must be confused with so many new words that does not make much sense to you. Let's take a look at a simple example so that you can better understand these new terminology.

There are two alleles for the gene that determines the seed color of peas. The yellow allele is dominant, so let's express the allele as Y. The green allele is recessive, so let's express the allele as y.

The 3 different genotypes that an individual can have are:

• YY : This means that the pea has yellow alleles on both homologous chromosomes. Because there are two of the same allele, this is homozygous.
• Yy: This means that the pea has one yellow allele and one green allele. Because there are two different alleles, this is heterozygous.
• yy: This means that the pea has green alleles on both homologous chromosomes. Because there are two of the same allele, this is homozygous.

• For the pea that has YY genotype: Since this individual has two dominant alleles, the dominant allele will be expressed. The phenotype will be yellow seeds.
• For the pea that has Yy genotype: This individual has one dominant allele and one recessive allele. The dominant allele is expressed even when it is paired with a recessive allele, so the dominant allele will be expressed for this individual as well. The phenotype will be yellow seeds.
• For the pea that has yy genotype: This individual has two recessive alleles. Since there is no dominant allele present, the recessive allele will be expressed. The phenotype will be green seeds.

Genetic Diagram

Now, we will learn how we can predict the phenotype and genotype of offspring of a cross using genetic diagrams.

In the following examples, there are two alleles for a gene that determines the height of pea plants: tall allele (dominant, T) and dwarf allele (recessive, t).

I. Crossing TT x tt

1. The first row of circles shows the genotype of the parents. The genotype of one parent is homozygous dominant (TT), and the genotype of the other parent is homozygous recessive (tt).
2. The second row of circles shows the possible gametes that are formed. The gametes formed during meiosis are haploids, so the gametes can carry only one allele from the parent.
3. Each colored line joining the second and third row shows the all the possible ways that one gamete from each parent combine during fertilisation. 
4. The third row of circles shows all the possible genotypes of offspring produced from the cross of a TT and tt parent. There is only one possible genotype possible: Tt
5. Since the Tt genotype contains a dominant allele, the dominant allele will be expressed. All the offspring produced will be tall.

This can also be done using the Punnett square:

1. Write down the gametes formed by one parent across the top, and write down the gametes formed by the other parent down the left side.
2. Fill in the empty space by writing down the genotype of offspring produced by the combination of each gamete.

II. Crossing Tt x Tt

1. In this case, both parents are tall and have a Tt heterozygous genotype.
2. Both parents can produce one gamete with a T allele and another gamete with a t allele.
3. If we look at the genotypes of offspring, there is one TT, two Tt, and one tt. This means that the genotype of the offspring produced from Tt x Tt will be 25% TT, 50% Tt, and 25% tt.
4. If we look at the phenotypes of offspring, the offspring with TT genotype will be tall. The offspring with Tt genotype will also be tall. Only the offspring with tt genotype will be dwarf. Therefore, the probability that the offspring is tall is 75%, and the probability that the offspring is dwarf is 25%. 

Punnett square for this looks like this:

Codominance

When neither allele is recessive and both alleles contribute to the phenotype of a heterozygote.

For example, there are two alleles for the color of petals of a flower: R (red) and r (white). 

There are three possible genotypes: RR, Rr, and rr.

There are also three possible phenotypes: red, pink, and white.

A typical example of codominance in humans is the ABO blood type. 

• The ABO blood type is determined by three alleles: I^A, I^B, and i
• I^A and I^B alleles are codominant alleles.
• Both I^A and I^B alleles are dominant over i allele.

Family Pedigree

A family pedigree can be used to see how a genetic disorder is inherited in a family.

Let's take a look at an example of a pedigree that shows the pattern of inheritance of a familial hypercholesterolemia (FH) within a family. Familial hypercholesterolemia (FH) is an inherited condition caused by a dominant allele. People with the condition have high levels of cholesterol in their blood. This increases the risk of dying from blocked arteries.

If Person A is heterozygous for FH, what is the probability of G and H having three children who all have FH?

To answer the question, we need to find out the genotype of G and H. 

• Since H is not affected by FH, it means that H is homozygous recessive.
• G can be either homozygous dominant or heterozygous. To figure out the exact genotype of G, we need to know the genotype of C (the mother of G).
• Since C is the daughter of the cross of heterozygote (A) and homozygous recessive (B), the genotype of C can only be heterozygous or homozygous recessive. (The Punnett square is shown below) Since we know that C is affected by FH, it means the genotype of C is heterozygous.
• Now, we know that G is also heterozygous since G is the offspring of a heterozygous (C) and homozygous recessive (D).
• Now, we know that G is heterozygous and H is homozygous recessive.
• The probability of a child born between G and H having FH is 0.5. The probability of all three children having FH is 0.5 x 0.5 x 0.5 = 0.125.

👇 Punnett square of a cross between heterozygous and homozygous recessive 👇

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