Unit 2: Genetics

Genes: the blueprint for proteins• Genetics: the study of how inheritable

characteristics are passed on from generation to generation.

• Gene: A section of the DNA that defines a certain characteristic.

The proteins produced by the cell determine the majority of the physical characteristics of the individual.

Genetic code: the chemical language through which the information needed to produce a complete human is transmitted

Genes determine which of 20 kinds of amino acids are to be linked together to make a protein molecule

DNA (deoxyribonucleic acid)

• The DNA code is made up of four special chemicals called bases

• These four bases are represented by C, G, T and A

• In humans, these chains are several million bases long

• Opposite every C on one DNA strand is a G on the other strand; opposite every T on one strand is an A on the other.

Protein Synthesis

Genetics Terms

• Characteristic: some observable feature that can appear in more than one form, such as plant height

• Trait: a variation that can exist for a characteristic eg tall or short

• Purebreed: an organism in which all offspring and their descendents have the same trait for a particular characteristic

• Hybrid: from the crossing of two purebreeds that have different traits

• Monohybrid cross: crossbreeding experiment that follows the inheritance of a single characteristic across one or more generations

Who was Gregor Mendel?

• Gregor Mendel (1822-1884) was an Austrian monk who realized that patterns of inheritance could be determined when observed over several generations.

• Mendel studied numerous characteristics of pea plants at his monastery, growing over 50 000 plants during a 10-year period.

• Mendel made careful records of all of his experiments and observations.

• Mendel’s insights led to the birth of the science of genetics

Common Edible Pea Plant

Mendel's Traits

Mendel’s Monohybrid Cross

• Mendel followed one characteristic at a time, through at least three generations

P generation: parent generation, usually purebred

F1 generation : the first filial generation. The offspring of the P generation. Usually hybrids.

F2 generation : the second filial generation. The offspring of the F1 generation are crossed.

- A dominant trait is fully expressed in the offspring, even in a hybrid

- A recessive trait is not expressed in a hybrid, meaning that the recessive trait is not visible it is masked by the dominant trait

Purebred yellow seeds, crossed with purebred green seeds

Probability of Inheritance• Probability is a

measure of the likelihood that one or another kind of outcome can happen, and is governed by the rules of chance.

• Mendel’s results did not show an exact 3:1 ratio, which shows that it is chance that determines the outcome.

Genes & Alleles

• Genes are passed from parent to offspring as the hereditary unit of information, as a length of DNA located in a particular place on the organism’s chromosomes eg eye colour

• Alleles are the possible versions of the gene. Eg blue or brown

Mendel’s Gene Hypotheses:1. The presence of different alleles is responsible for the variation in

the appearance of the organism.2. An organism always has two genes present for each

characteristic (one from each parent).3. If the alleles on both of the genes are the same, then the

organism is purebred4. If the two alleles are different, then the organism is a hybrid. One

trait will be dominant and one will be recessive. Only the dominant trait is expressed.

Representing Alleles• The presence or absence of a particular trait has no influence on

other traits Mendel called this the Law of Independent Assortment

• Letters are used to represent different alleles of a gene:- the allele for the dominant trait is T = tall plant allele- the allele for the recessive trait is t = short plant allele

P generation TT (tall) X tt (short)

F1 generation all Tt (tall plants)

F2 generation

3 tall: 1 short

Punnett square

T t

T TT Tt

t Tt tt

Physical and Genetic types

• Phenotype refers to the appearance of a particular characteristic in an organism. Phenotype describes which trait is expressed.

• The phenotypic ratio compares the number of each phenotype that is expressed in the offspring.

• Genotype refers to the genetic makeup of an organism. The two genes may have matching alleles, called homozygous

(TT or tt), or non-matching alleles, called heterozygous (Tt). TT: homozygous dominant tt: homozygous recessive Tt: heterozygous The genotypic ratio compares the number of each genotype

that is expressed in the offspring.