What is genetics?

• GENETICS is the branch of Biology that studies heredity.

• HEREDITY is… the passing on of characteristics from parents to offspring.

Traits and Genes• TRAITS are CHARACTERISTICS that are

INHERITED.• Genes control the expression of traits. • A gene is a segment of DNA that controls

the production of a protein.

What did Mendel observe and conclude?

• Mendel observed that there were DIFFERENT FORMS OF A TRAIT (controlled by a gene) called alleles.

• Each parent donated one allele for each trait, which meant that each offspring had two alleles for each trait.

GREGOR MENDEL

• Gregor Mendel was the first person to study genetics.

• Mendel was a monk from Austria.

1. He did his genetics experiments using pea plants.

2. He crossed (mated) two pea plants that had certain traits.

3. He then planted their offspring and observed what traits they expressed.

4. He carefully selected each plant that he used and carefully reviewed the results.

P1 and F1 generations

• In a cross (mating) between organisms, the parent generation = P1 generation.

• The first generation =

F1 generation.

• The second generation = F2 generation.

Genotype and Phenotype

• The phenotype an organisms appearance and behavior (how the trait expresses itself).

• The genotype for an offspring or parent is the gene combinations or the set of two alleles that they have for each trait.

Homozygous and

Heterozygous

• If the alleles are the same for a trait then the organism’s genotype is homozygous for a certain trait.

• If the organism has two different alleles for each trait then its genotype is heterozygous (these are often called hybrids).

What did Mendel observe and conclude?

• Mendel noticed that some traits masked the expression of other traits.

• For example, offspring from a cross between tall and short pea plants were all tall.

Dominant and Recessive Alleles• Mendel came up with

his rule of dominance : an allele that masks the expression of another allele is dominant.

• The dominant allele is usually capitalized.

• The other allele is recessive and it is written lower case.

How it works….

• For example, the allele for a tall pea plant is dominant over the allele for short pea plants.

• The dominant allele is expressed when the organism is:– homozygous =TT or – heterozygous =Tt

• The recessive allele for a trait is expressed only when the organism is – Homozygous for recessive= tt

Genotype and phenotype questions.

• For example, for the height trait, if tallness (T) is dominant over shortness (t) in pea plants, what is the genotype of a homozygous tall pea plant?

– _______________________

• What is the phenotype of a pea plant with the genotype Tt for its height?

– ________________________

• What is the phenotype of a pea plant with the genotype tt? What do we call this genotype?

– _______________________

Punnett Squares!Punnett squares (devised by Reginald Punnett) give us a shorthand way to determine the expected genotypes and phenotypes of the offspring that result from a cross.

What can I do with a Punnet Square?

• FROM THIS, YOU CAN CALCULATE:– the numbers of each genotype and

phenotype. – the probability of an offspring having a

certain phenotype. – And how many offspring should exhibit a

certain form of each trait by multiplying the number of offspring by the probability that they will have a certain trait.

Punnett Square problem

• In pea plants, tallness (T) is dominant over shortness (t) with respect to height.

• If two heterozygous pea plants (each have genotype Tt) are crossed, what are the expected numbers of each phenotype and genotype? ________

Continued….• What is the probability that the

offspring pea plant will be tall? _________

• What percentage of the offspring are tall? _________

• What is the ratio of tall offspring to short offspring? ________

• If the pea plants produce four offspring, how many of them will be short?___________

Do we see these same results in nature?

•In Mendel’s experiments, he saw phenotypic ratios that very closely matched the THEORETICAL result of the Punnett squares (invented after his death).

•His results would have been closer to the expected results if he had used more pea plants and created more offspring from them.

LAW OF INDEPENDENT ASSORTMENT

• The law of independent assortment states that allele pairs separate independently during the formation of gametes. – Therefore, traits are transmitted to

offspring independently of one another.

• This is shown in metaphase I of meiosis due to the fact that the homologous chromosomes line up independently of one another and then separate.

• These laws do not always hold true in all cases, however they provide a foundation for the basis of our understanding of genetics.

The Law of Segregation• The law of segregation states that allele

pairs separate or segregate during gamete formation, and randomly unite at fertilization.

• This makes sense according to meiosis because homologous chromosomes separate during anaphase I of meiosis.

• Thus, each parent gives one allele, for a given trait, to its offspring.

• This is due to the fact that the parent’s gametes only contain one copy of an allele for a certain trait.

DIHYBRID crosses• Mendel also performed

experiments where he looked at the variation of two traits amongst pea plants.

• He crossed pea plants that differed from each other in two traits.

Dihybrid crosses• Mendel carefully noted his results and the numbers

of each offspring with certain phenotypes. • From these, he was able to create his two laws of

heredity.

Dihybrid punnet square

• You must then make the possible allele combinations that the parent’s gametes can have.

• There will be four possible allele combinations for dihybrid crosses.

• For example, the genotype TtRr would make the following gametes: – TR, Tr, tR, tr. – What gametes would the genotype TTrr make?

________

Dihybrid cross problem• In pea plants, tallness (T) is dominant over shortness (t)

with respect to plant height. • Yellow seeds (Y) are dominant over green seeds (y) with

respect to seed color. • If you cross two pea plants that are heterozygous tall and

heterozygous for yellow seeds (TtYy), what are the expected genotypes and phenotypes?

Dihybrid Cross Problem

• What is the probability that the offspring will be tall and have yellow seeds? What is the probability that an offspring will be short and have yellow seeds?______

• If the plants produce 16 offspring, how many of them will be short and have green seeds? _____

• How many will be tall and have green seeds?_________