Mendel’s Laws of Heredity

Mendel’s Laws of Heredity What is genetics: Genetics – the branch of biology that studies heredity. Heredity – the passing on of traits from parents to offspring. Traits – Characteristics that are inherited.

The work of an Austrian monk named Gregor Mendel was particularly important in the understanding of inheritance. • Mendel was born in 1822 in what is now the Czech republic. • He studied to be a priest and spent several years studying science and math at the University of Vienna.

Mendel spent the next 14 years of his life working in the local monastery and teaching high school. • At the monastery, Mendel was in charge of gardening and spent most of his time tending to and studying the plants around him.

Mendel paid close attention to the common garden pea plants that grew at the monastery. • These pea plants reproduced sexually, meaning they had both male and female sex cells. Gametes – male or female sex cells.

Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms: 1. flower color is purple or white 2. seed color is yellow or green 3. flower position is axil or terminal 4. pod shape is inflated or constricted 5. stem length is long or short 6. pod color is yellow or green 7. seed shape is round or wrinkled

He preformed experiments with these pea plants by forcing plants with different traits to pollinate each other. • These experiments are called genetic crosses.

Mendel’s Monohybrid Crosses. • In his first experiment, Mendel decided to cross pollinate two plants that had different seed color. • One of the plants had all green seeds and one of the plants had all yellow seeds. • The crossing of one trait is called a monohybrid cross.

Mendel expected the first generation of new plants to have a mixture of green and yellow seeds. • These first generation plants however all had yellow seeds. • Mendel then decided to cross two of the offspring in the first generation and see what type of seeds he would find in the 2nd generation.

To his amazement Mendel found that in the second generation, the plants had both green and yellow seeds, and the ratio of the seeds per plant was three yellow seeds to every one green (3:1). • After careful mathematical calculations and a lot of guesswork, Mendel made the largest breakthrough in modern science.

Mendel concluded that each trait for seed color was represented by something that the plant carried. • Because every plant had a “mom” and “dad”, Mendel concluded that each trait had two parts, and that it is the combination of these parts that give the organism it’s traits.

He called these parts Alleles. Alleles – Gene form, Yory, for each variation of a trait of an organism. Dominant – visible, observable trait of an organism that masks a recessive form of a trait. Recessive – A hidden trait of an organism that is masked by a dominant trait.

Law of Segregation • After noticing that the allele for green seeds reappeared in the second generation, Mendel concluded that the two alleles for each trait must separate when gametes are formed. Law of Segregation – In other words, A parent only passes one form of a gene or allele (i.e. either A or a but not both) at random to each offspring.

Genotype and Phenotype Phenotype – the way an organism looks and behaves. Physical characteristics of an organism. Genotype – the gene combination of an organism.

Homozygous – when an organisms two alleles for a trait are the same. Heterozygous – when an organisms two alleles for a trait are not the same.

Mendel’s Dihybrid Crosses Studying Two Traits at once: Mendel also did a type of cross where two traits were followed at one time - a Dihybrid Cross.

AA or Aa = purple; aa = whiteBB or Bb = tall; bb = short

Law of Independent Assortment – Genes of different traits are inherited independently from each other. • In other words, green peas don’t always come from tall plants, they may be yellow and tall. • Segregation and Independent Assortment of Alleles when the alleles are on separate chromosomes.

Punnett Squares: in 1905, Reginald Punnett, an English biologist devised a short way to show the expected proportions of possible genotypes in the offspring of a cross.

Your turn! Try these on your own paper. Problem 1: (a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?

Answer: Genotype: Ww (all children will be heterozygous) Phenotype: Widow's peak (phenotype of all children) w w W Ww Ww W Ww Ww

(b) Suppose one of their children (Ww) marries someone who is also heterozygous (Ww). What type of hairline will their children have?

Problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles?

Problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no

Problem 4: Jane and John are expecting a baby and know that they are both carriers (ie heterozygous) of cystic fibrosis (Cc). What is the probability that their child will have cystic fibrosis (cc)? What is the probability that their child will be a carrier of cystic fibrosis?

Mendel’s Laws of Heredity