DNA and Inheritance

Biology 30 Genes & Heredity Your biological traits are controlled by genes, which are located on the chromosomes that are found in every cell of your body. Only one copy of each gene is on a chromosome. Genes are like books; they may or may not be read by the chemical machinery of the cell.

Your traits are a result of the interactions of the genes from both parents. Although you contain half the genes from each parent, your genes and traits are uniquely your own. There are over eight million combinations possible from the 23 chromosomes you inherit from your mom and 23 you inherit from your dad. The passing of traits from parents to offspring is called heredity.

Pioneer of Genetics: Gregor Mendel Gregor Mendel, an Austrian monk, conducted some simple but significant experiments in his monastery garden. (1822-1884).

Mendel’s work with garden peas not only explained the mechanism of gene inheritance for plants, but also provided a basis for understanding heredity.

Why did Mendel work with the garden pea? 1) Garden peas have a number of different characteristics that can be expressed in one of two ways. (ex. yellow vs. green pea) 2) It was easy for Mendel to go about his experiments, because of the way the plant reproduces.

Mendel fertilized plants by cross-pollination rather than self-fertilization.

Mendel’s Characters The seven characteristics that Mendel studied in peas

Mendel’s Experiments Crossed the pollen from a plant that produced round seeds with the eggs of the one that produced wrinkled seeds, the offspring were round.

Crossed the pollen from a plant that produced wrinkled seeds with the eggs of the one that produced round seeds, the offspring were round.

Mendel then crossed the F1 hybrids Mendel then crossed the F1 hybrids together to produce an F2 generation. The recessive trait “reappeared”. F1 F2

He found a 3:1 ratio of dominant to recessive phenotypes. 3 round : 1 wrinkled 1/4 homozygous dominant 1/2 heterozygous 1/4 homozygous recessive

Rr r R R RR Rr Rr r rr Rr 1/4 homozygous dominant 1/4 homozygous recessive r rr Rr

He repeated the procedure for other characteristics (eg. flower colour). He discovered that one trait dominated another, whether the sex cell came from a female or a male part of the plant. Mendel reasoned that things called “factors” controlled the traits of a plant. These factors were later termed genes.

Dominant genes – Determine the expression of the genetic trait in the offspring. (Capitalize this letter). Recessive genes – Are “masked” by dominant genes. (Lower case letter).

Mendel’s Laws of Heredity First Law (Law of Unit Characters) Inheritance is governed by genes that exist in the individual and are passed on to offspring. These factors (genes) occur in pairs, one gene comes from the female and one gene comes from the male. The alternate forms of the same genes are known as alleles.

Second Law (Law of Dominance) One factor, or gene, masks the effect of another. This process is known as the principle of dominance. The dominant expression is seen and the recessive gene is not seen (remains hidden).

Third Law (Law of Segregation & Recombination) A pair of factors (genes) segregate/separate during the formation of sex cells (meiosis). As a result, each parent can only contribute one member (allele) of a pair of genes to their offspring.

Genetic Terms Define the following terms: Genotype – Phenotype – Homozygous (“purebred”) – Heterozygous – Alleles –

Monohybrid Cross The combining of single contrasting traits: eg. Crossing a tall plant (TT) with a short plant (tt) a special chart called a Punnett square, helps geneticists organize the results of a cross between the sex cells of two individuals from a Punnett square, we can predict the genotypes & phenotypes of the offspring:

Draw a Punnett square for a cross between a heterozygous round-seed plant and a wrinkled-seed plant. Rr rr r Rr rr r In the F1 generation ½ are round ½ are wrinkled

Rr r R R RR Rr Rr r rr Rr 1/4 homozygous dominant 1/4 homozygous recessive r rr Rr

Draw a Punnett square for a cross between two heterozygous round-seed plants. RR Rr R Rr rr r In F1 generation ¾ are round ¼ wrinkled

Draw a Punnett square for the cross between a heterozygous purple flower with a white flower. Ww ww w Ww ww w In the F1 generation ½ are purple ½ white

Phenotypic Ratio: 50% of flowers are purple 50% of flowers are white Genotypic Ratio: 50% of flowers are Ww 50% of flowers are ww

Test Cross A test cross is often performed to determine the genotype of a dominant phenotype. When would we need to know this? There would be some offspring produced that have white wool and some with black. eg. A sheep farmer wants to ensure that all of his flock will have white hair (black wool is brittle & hard to dye). He chooses a white ram to mate with the flock. What if the ram is heterozygous?

To ensure that the ram is not, a test cross can be performed to ensure that the ram is homozygous for the white phenotype A test cross is always performed between the unknown genotype and a homozygous recessive (known) genotype.

Possible outcomes: If 50% of the offspring are black and the other half white, the unknown genotype must have been heterozygous (Ww). If 100% of the offspring are white, then the unknown genotype must be homozygous dominant (WW).

Mendel showed that by performing a. test cross with the homozygous Mendel showed that by performing a test cross with the homozygous recessive, the genotype could be determined. YY x yy Yy

Yy x yy yy Yy Y y This test cross produces 1/2 dominant and 1/2 recessive phenotypes