Unit 3 – Genetics Mendelian Genetics

What is genetics? Genetics is the branch of biology dealing with the principles of variation and inheritance in plants and animals. Heredity is the passing of traits (characteristics), such as brown colour hair, from one generation to the next. The traits that are passed on to the next generation, are said to be inherited.

Gregor Mendel was the father of genetics He used garden pea plants to explain inheritance of traits from one generation to the next

Mendel’s first experiment was carried out on pea plants. He first had to produce purebred plants. Purebreds are organisms that descend from ancestors of a particular type. Mendel produced the purebred plant through selective breeding. He would breed tall plants with tall plants until they only produced tall offspring. He did the same for short plants. Tall and short are examples of traits

Once he had the purebreds, he called them the P Generation (aka parent genernation) He then crossed (bred) the tall and short plants of his P generation together. He labeled the offspring of that cross the filial generation or the F1 Generation. He called the plants of the F1 generation hybrids because they were a cross between tall and short plants

This is called a monohybrid cross because it only involves one trait This is called a monohybrid cross because it only involves one trait. In this case it is plant height.

Mendel’s next step: He allowed plants from the F1 generation to self fertilize. Tall F1 x Tall F1 F2 generation : One of every 4 plants was short. The rest were tall. The trait for shortness had reappeared !

His explanation: Distinct units of heredity or factors( now known as genes) were responsible for inherited traits. Two factors controlled any single trait (ie.) one factor produced tall and one factor produced short. The tall (T) factor was dominant. The short (t) factor was recessive

Mendels F1 generation

A dominant trait is a characteristic that always appears in an individual if present.   A recessive trait is a characteristic that is not usually expressed in an individual.

Generally, the dominant factor only appeared in F1 generation and the recessive factor appeared in F2. Two letters are necessary to describe each combination of alleles (dominant or recessive form which a gene may take). The combination of alleles is called the genotype.

Genes are specific portions of a DNA molecule that determine the characteristics of an organism. They may dominant (ie.) determine the expression of genetic trait in an offspring. It prevents the expression of the recessive trait. It is represented by an upper case letter.

They may be recessive (ie. ) overruled by the dominant gene They may be recessive (ie.) overruled by the dominant gene. It is masked by the dominant gene. It is represented by a lower case letter; (eg.) brown eyes (BB or Bb) ; blue eyes (bb).

Homozygous : alleles are the same — BB, bb Heterozygous : alleles are different — Bb The physical expression of a trait is called the phenotype ; (ie.) brown eyes , blue eyes. An organism can have the same phenotype (brown eyes) but have a different genotype (BB or Bb).  

Mendel’s First Law: The factors for a pair of alternate characters are separate and only one may be carried in a particular gamete; genetic segregation. Modern form: alleles segregate in meiosis   Mendel’s Second Law: The inheritance of a pair of factors for one trait is independent of the

simultaneous inheritance of factors for other traits, such factors assorting independently as though there were no other factors present ; later modified by the discovery of linkage). Modern form: unlinked genes assort independently

Mendel’s Principles : A) SEGREGATION refers to the separation of paired genes during meiosis. The two factors of each pair segregate and act as separate gametes. These factors that occur in pairs are separated from each other during gamete formation and are recombined during fertilization. This is the LAW OF SEGREGATION.   B) DOMINANCE refers to a dominant gene of a trait masking the expression of another gene; (ie.) one gene masks the effects of another. When an organism is hybrid for a pair of contrasting traits, only the dominant trait can be seen in the offspring. This is the LAW OF DOMINANCE. C) INDEPENDENT ASSORTMENT : Genes are located on chromosomes. During gamete formation , chromosomes sort out such that one chromosome from each pair goes into a gamete. Chromosomes separate and sort out independently. This is the LAW OF INDEPENDENT ASSORTMENT.

LAW OF SEGREGATION. B) DOMINANCE refers to a dominant gene of a trait masking the expression of another gene; (ie.) one gene masks the effects of another. When an organism is hybrid for a pair of contrasting traits, only the dominant trait can be seen in the offspring. This is the LAW OF DOMINANCE.  

C) INDEPENDENT ASSORTMENT : Genes are located on chromosomes C) INDEPENDENT ASSORTMENT : Genes are located on chromosomes. During gamete formation , chromosomes sort out such that one chromosome from each pair goes into a gamete. Chromosomes separate and sort out independently. This is the LAW OF INDEPENDENT ASSORTMENT.