Mendelian Genetics

Chapter 11

Gregor Mendel – “Father of Genetics”, first Geneticist, one of the most important scientists in history

Austrian Monk (1822-1884

Worked with ordinary garden peas

Used science and math skills learned from University.

Genetics – the scientific study of heredity

Mendel’s Peas – background information

• The anther (♂, male) produces sperm• The ovary (♀, female) produces eggs• Sperm + Eggs: FERTILIZATION• The peas Mendel started with were TRUE-

BREEDING.

Gametes

More Peas

True-breeding: parents always have offspring identical to themselves when allowed to self-pollinate. (Is self-pollination sexual or asexual reproduction?)

For example: Plants with white flowers produce only offspring with white flowers.

Important terms

Contrast with true-breeding

Trait = specific observable characteristic

Mendel produced hybrids

Hybrid The offspring produced

by parents with differing forms of a trait.

Some genetics notation Parent generation = P F1 = first generation of

offspring from a cross, (F2 = second generation, etc.)

Mendel’s crosses…results

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Mendel’s crosses…results

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Genes Mendel eventually concluded that traits are passed from one

generation to the next.

Today, we know that… Traits are determined by GENES

Chemical factors that determine a trait. Consist of DNA sequences

Alleles - Different forms of a gene Ex. Eye color is a trait, you could have 2 blue eye color

alleles, you could have 2 brown eye color alleles, you could have one of each.

You get one allele from each parent for each trait.

Mendel did not know anything about genes, DNA, chromosomes, meiosis, etc.

Dominant and Recessive Genes Principle of Dominance: Some alleles are

dominant and others are recessive.

Dominant: An allele that is expressed (visible) whether it is the only version of the gene present or when in combination with a recessive allele.

Recessive: An allele ONLY expressed when dominant allele is NOT present

Symbols to represent Dominant Genes If the symbol “T” represents the dominant

allele (or form of the gene), then… it doesn’t matter whether both of the alleles

an individual has are dominant (TT) or… there is just one copy of the dominant allele

(Tt). Either way, the individual looks the same (it

has the dominant trait).

Symbols to represent Recessive Genes If the symbol “t” represents the recessive

allele (or form of the gene), then… both of the alleles an individual has must be

recessive (tt) to have the recessive trait. The presence of one dominant allele (T)

hides the recessive allele (t) when there is one of each (Tt).

Important vocabulary

Homozygous two identical alleles for a trait. (ex. SS or ss) TRUE-BREEDING for a trait

Heterozygous two DIFFERENT alleles for a trait (ex. Ss) HYBRID for a trait

Phenotype physical characteristics, appearance

Genotype genetic make up, the information in DNA

End part one

Mendelian Genetics

Chapter 11

Segregation

SEGREGATION Separation of alleles during gamete formation

(meiosis) Gamete

Sex cells carry a single copy of each gene (ex. The allele

for short OR the allele for tall, not both)

Genetics and Probability

Probability likelihood that a particular event will occur. For example: What is the probability of rain

tomorrow? How likely is it that a flipped penny will land

heads-up? Tails-up? Probability is used to predict the outcomes of

genetic crosses.

Steps to a setting up a Punnett Square 1. Identify parental genotypes for the cross

(ex. TT, tt, Tt)2. Draw the Punnett Square--a grid with each

set of parental alleles on its own side.3. Separate the alleles (why?)4. Match alleles from rows and columns to

make offspring genotypes.5. Identify the kinds (and their numbers) of

offspring you have.

Gametes (sperm or eggs) are haploid (one set of chromosomes)

Steps to a setting up a Punnett Square: An Example 1. Both parents are heterozygous for plant

height, with tall being dominant to short.

2. Describe their offspring.

Probability

Look at the cross we just did. How many offspring have two alleles for

tallness (TT)? Two for shortness (tt)? Both (Tt)?

Probabilities ¼ or 25% have two alleles for tallness. ¼ or 25% have two alleles for shortness. ½ or 50% have one allele for tallness and

one for shortness. We write this in terms of a ratio 1TT: 2Tt: 1tt This ratio of allele combination types is called

the genotypic ratio.

Probabilities

How many plants will be tall and how many will be short?

We also write this in terms of a ratio (3 tall: 1 short).

This ratio of physical types is called the phenotypic ratio.

3 tall (75%), 1 short (25%)expressed as a ratio, 3:1 Why?

1 homozygous for tallness (TT) and the 2 heterozygous plants (Tt) You lump these together because you cannot tell

any of these offspring apart for this trait—TT offspring look exactly like Tt offspring.

1 homozygous recessive (tt)

This is exactly what Gregor Mendel did in his pea experiments.

A brief explanation… All of the reproduction we will be discussing

regarding Mendel’s garden peas will be sexual reproduction (a new organism formed by the joining together of a male and a female gamete).

However, pay close attention to who the parents are — sometimes there is a cross between two different parents (usually with contrasting forms of some trait, like flower color or height). Other times, the peas are allowed to do what comes naturally to peas—self-pollination (the same plant fertilizes itself).

Sexual Reproduction in Plants

Female (♀) sexual organs

Male (♂) sexual organs

Cross-pollination – either by wind, insect or geneticist Pollen

(containing male gametes)

Self-pollination

Convenient, but boring

Mendel’s first crosses The parent

generation (P) is a combination of two different pure-breeding types

The first generation (F1) produced only TALL plants

The second generation (F2 ) produced tall and short plants

Mendel’s Monohybrid cross

Tall allele/Tall allele

TT short allele/short allele

tt

Predictions

Mendel found that the approximately three dominant to one recessive ratios showed up consistently.

Probabilities predict average outcomes for a LARGE number of events, not exact outcomes.

Flip a coin twice Heads once, tails once Heads twice Tails twice

Flip a coin many times, likely to get very close to a 50:50 ratio

Predicting Genetics

Genetics is similar Larger numbers result in closer to expected

values. This explains why humans don’t always have

the same number of male children and female children despite the fact that the odds of any one child’s gender is 50% female, 50% male.

Mendel’s Principle Independent Assortment

Genes for different traits segregate independently during gamete formation

Accounts for much of the genetic variation in living things Usually, all combinations of traits are possible, ex. Tall round seeds, purple flowers

Short round seeds, white flowers

Short wrinkled seeds purple flowers

Tall wrinkled seeds white flowers

Etc.

Summary of Mendel’s Principles

Genes determine inheritance. Genes are passed from parent to offspring.

Some genes may be dominant and others recessive

Adults (sexually reproducing) have two copies of each gene—one from each parent. They segregate during gamete formation.

Alleles for different traits assort independently (Independent Assortment)