LifeLine

When you think of genetics today, you might picture DNA or chromosomes. But the story of genetics actually begins in the 19th-century with Gregor Mendel, who carefully observed pea plants. His work laid the foundation for everything we know about heredity today.

Alleles & Genetic Code

• Allele: Two different or alternate versions for the same gene are called an allele.
  • An individual in a lifetime will have one pair of alleles. One from mother, one from father.
• Dominant Allele: The allele which can express itself even in the presence of another/different allele for the same gene.
  • A dominant allele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for black fur in dogs is dominant. Only one copy of this allele is needed in order to have black fur. Two copies of the dominant allele will still produce black fur. 
• Recessive Allele: The allele which can express itself only when present in pairs.
  • A recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene. Recessive alleles are represented by a lowercase letter, for example, a. The allele for brown fur is recessive. A dog needs two copies of this allele to have brown fur.
• Homozygous Individual: Two identical alleles for a particular gene. 
• Heterozygous: Two different alleles for a particular gene. 
• Genotype: The genetic constitution/composition/makeup of an individual passed on from generation to generation. It also refers to the pair of alleles inherited in an individual for a gene.
  • The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
• Phenotype: It refers to the observable characteristics/traits such as height, blood group, eye color. A person’s phenotype is determined by both genotype and environmental conditions. 

Example Of Homozygous & Heterozygous

Blood group 

Mom (AA) – Homozygous

Dad (BB) – Homozygous 

Child (AB) – Heterozygous 

Mom (YY) – Homozygous

Dad (yy) – Homozygous

Child (Yy) – Heterozygous

Example Of Phenotypes & Genotype

Genotype → Yy

Phenotype →Yellow Coloured Seed

Genotypic Ratio – 1 (yellow):2 (yellow):1 (Green)

Phenotypic Ratio – 3 (Yellow):1 (Green)

Mendel’s Laws Of Inheritance

• Law of Dominance – This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits.
• Law of Independent Assortment – Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together.
• Law of Segregation – The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization. This is also known as Mendel’s third law of inheritance.

General

• F1 (Grandparents), F2 (Parents) – Filial 
• The generation f2 is also showing the traits being carried from the parents of the F1 generation. This states that the characteristics are passed from one generation to another. 
• How do we know if it’s heterozygous or homozygous?
  • Pure breeding: Selecting the traits you would like to incorporate. 
  • Pure lines: Descendants obtained after inbreeding for several generations.
  • When you are crossing your desired traits done during inbreeding the chances to obtain your desired traits are increased from F1 generation to the subsequent generations

• Monohybrid Cross: When you are considering only one gene/characteristic for a cross it is a monohybrid cross. 
• Dihybrid: A cross where two characteristics or genes are considered. 
• Two contrasting characteristics –  Crossing them together (eg. seed shape, seed color)
• Autosomal – A chromosome that isn’t a sex chromosome 

Punnett Square 

Monohybrid crosses

Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. This shows which possible offspring combinations could be produced, and the probability of these combinations occurring.

Example 

The height of pea plants is controlled by a single gene which has two alleles: tall and short.

The tall allele is dominant and is shown as T.

The small allele is recessive and is shown as t.

	T	T
t	Tt	Tt
t	Tt	Tt

If you add the combinations into each box you see that all the possible offspring have the same allele combination – Tt.

Note: you should always write the dominant allele first.

This means that all (100%) of the offspring produced will be tall.

Predicting Genotypes & phenotype With Punnett Squares

Mendel developed the law of segregation by following only a single characteristic, such as pod color, in his pea plants. In a monohybrid cross, such as the one in the picture the Punnett square shows every possible combination when combining one maternal (biological mother) allele with one paternal (biological father) allele. In this example, both organisms are heterozygous for flower color Bb (purple). Both plants produce gametes that contain either the B and b alleles. If the gametes from both parents contain the dominant alleles, the resulting plant will be homozygous dominant and have purple flowers. If the gametes from both parents contain the recessive alleles, the resulting plant will be homozygous recessive and have white flowers. If the gamete from one parent contains the dominant allele and the gamete from the other parent contains the recessive allele, the resulting plant will be heterozygous and have purple flowers. The probability of any single offspring showing the dominant trait is 3:1, or 75%.