Mendel's Genetics

The Terms

Gene – composed of DNA. The sequence of bases in DNA will code for the production of protein (trait). Allele – one of the two genes responsible for a specific trait; symbolized by letters T, t

Locus – the position of the gene on the chromosome Gene Pool – all the genes of a population Dominant – the “stronger” gene shown up in the offspring (capital letter) Recessive – the “weaker” gene that doesn’t necessarily show up in the offspring (lowercase)

Homozygous – have two of the same alleles (pure) for the trait TT, tt Heterozygous – have two different genes for the trait (hybrid) Tt Tt will tend to show stronger trait

Homozygous Heterozygous

What is Homozygous? Homozygous (pure) Having a pair of identical genes for a trait Ex: Tall (T) = TT, or tt

What is Heterozygous? Heterozygous (hybrid) Having two different genes (alleles) for a trait Ex: Tt

Phenotype – physical appearance of the individual Ex: brown hair, blue eyes, widow’s peak, etc. Genotype – the genetic makeup of the individual Ex: Is the person homozygous dominant, homozygous recessive, or heterozygous

Phenotype

Phenotype Phenotype – the physical and observable characteristics Ex: Tall, short, brown eyes, blond hair, freckles

Genotype

Genotype Genotype – the genetic makeup of an individual Ex: TT, Tt, tt

Gregor Mendel

Gregor Mendel Gregor Mendel (1860’s) father of genetics He used pea plants because Few Traits Easy to Grow Had a lot of offspring Reproduce quickly He came up with a couple of laws for determining heredity

Mendel’s Laws we will be discussing are: The Law of Dominance The Law of Segregation The Law of Independent Assortment

The Law of Dominance

Law of Dominance In a cross between 2 pure contrasting traits (tall vs. short), only one of these traits appear in the next generation This is called the DOMINANT TRAIT The one that does not appear is called the RECESSIVE TRAIT (Tongue-curling is dominant)

LAW OF DOMINANCE

Attached Earlobes are Dominant and free earlobes are Recessive                                                                                   Attached Earlobes are Dominant and free earlobes are Recessive

Widow’s Peak is Dominant and no widow’s peak is Recessive                                          Widow’s Peak is Dominant and no widow’s peak is Recessive

A Fruit Fly with red eyes is Dominant and white eyes is Recessive

The Law of Segregation

History: Mendel worked with pea plants and selected seven traits to study that each occurred in two different forms. For instance, one trait he studied was pod color. Some pea plants have green pods and others have yellow pods. Since pea plants are capable of self fertilization, Mendel was able to produce true-breeding plants. A true-breeding yellow-pod plant for example would only produce yellow-pod offspring. Mendel then began to experiment to find out what would happen if he cross-pollinated a true-breeding yellow pod plant with a true-breeding green pod plant. He referred to the two parental plants as the parental generation (P generation) and the resulting offspring were called the first filial or F1 generation

Law of Segregation When Mendel performed cross-pollination between a true-breeding yellow pod plant and a true-breeding green pod plant, he noticed that all of the resulting offspring, F1 generation, were green.

Law of Segregation He then allowed all of the green F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 3:1 ratio in pod color. About 3/4 of the F2 plants had green pods and about 1/4 had yellow pods.

Law of Segregation: From these experiments Mendel formulated what is now known as Mendel's law of segregation. This law states that allele pairs (genes) separate or segregate during gamete formation (meiosis), and randomly unite at fertilization

The Law of Independent Assortment

Independent Assortment This principle states that two or more pairs of alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization.

Mendel performed dihybrid crosses in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring (Figure A) or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

Punnett Square

The Punnett Square Determines the probability of obtaining various results in genetic crosses Ex: cross a heterozygous tall (Tt) and a heterozygous tall (Tt)

Cross 2 Heterozygous Traits:

Cross a homozygous freckles with heterozygous freckles F = Freckles f = no freckles

Genotypes: 50% homozygous dominant 50% heterozygous Phenotypes: 100% Freckles

Monohybrid Cross Crossing one trait Ex: Crossing height

Cross a heterozygous freckles with heterozygous freckles F = Freckles f = no freckles

Genotypes: 25% homozygous dominant 50% heterozygous 25% homozygous recessive Phenotypes: 75% Freckles 25% No Freckles

Dihybrid Cross Crossing 2 traits Ex: crossing height and color

Backcross

Backcross – Is done to determine the genotype of an organism Unknown genotype x Pure recessive B = black b = white

Incomplete Dominance

Incomplete Dominance PINK With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink). Ex: Cross a pure red snapdragon (RR) and a pure white snapdragon (WW) (notice that only capital letters are used) Offspring Phenotype: ____________________ Neither trait dominates the other, which is why it is called incomplete dominance PINK

Cross a pink snapdragon (RW) with a pink snapdragon (RW) R = Red W = White Results:

Chickens

Andalusian Chickens

Codominance

Codominance In Codominance, both of the dominant traits appear together in the phenotype Ex: Roan cattle Cross a homozygous red coat cow (RR) and a homozygous white coat cow (WW)

Roan Cattle

Cross a homozygous red cow (RR) with a homozygous white cow (WW) R = Red W = White Results:

Multiple Alleles

Multiple Alleles If there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population We call this “multiple alleles” Ex: Human blood type Blood type exists as four possible phenotypes: A, B, AB, and O There are 3 alleles for the gene that determines blood type: IA = Type A IB = Type B i = Type O Note that according to the symbols used, that the allele for “O” (i) is recessive to the alleles for “A” and “B”

Blood Type: Cross a man with AB blood (IAIB) and a woman with type O blood (ii)

Blood Type: A man with AB blood is married to a woman with AB blood. What blood types will their children be and in what proportion?

What Determines Gender?

Gender Chromosomes determine everything from hair color and eye color to gender. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. These chromosomes determine gender.

Males determine Gender? Sperm cells in humans and other mammals contain one of two types of sex chromosomes. They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome. Therefore, the sperm cell determines the sex of an individual If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

Karyotype of a normal male with 22 pairs of autosomes and one pair of sex chromosomes.