Introduction to Genetics

Chapter 11

11- 1 The Work of Gregor Mendel

• Every living thing – plant or animal, microbe or human being – has a set of characteristics inherited from its parents

• Since the beginning of recorded history, people have wanted to understand how that inheritance is passed from generation to generation

Genetics• The scientific study of heredity

Gregor Mendel• Austrian Monk• Born 1822 in Czech

Republic• Worked at monastery and

taught high school• Tended the monastery

garden• Grew peas and became

interested in the traits that were expressed in different generations of peas

True breeding• If allowed to self pollinate they

would produce offspring identical to themselves

• He was also able to cross breed peas for different traits

Genes and Dominance• Mendel studied seven different

pea plant traits

• Each trait he studied had a contrasting form

Pea Plant Traits

Genes and Dominance• The offspring of crosses between

parents with different traits are called Hybrids

• When Mendel crossed plants with different traits he expected them to blend, but that’s not what happened at all.

• All of the offspring had the character of only one of the parents

Mendel drew two conclusions

1. Inheritance is determined by factors that are passed from generation to generation – today we call these factors genes

Alleles• Different forms of a gene

Mendel’s 2nd conclusion

2. The Principal of Dominance

• Some alleles are dominant and some are recessive

dominant• Covers up the recessive form

Ex.) T = tall

recessive• Gets covered up in the presence

of a dominant allele

Ex.) t = short

Segregation• Mendel wanted to answer another

question

Q: Had the recessive alleles disappeared? Or where they still present in the F1 plants?

• To answer this he allowed the F1 plants to produce an F2 generation by self pollination

P1 Parental

Tall Short All Tall

F1 F2

3 tall : 1 short

75% tall

25% short

The F1 Cross• The recessive traits reappeared!

• Roughly 1/4 of the F2 plants showed a recessive trait

Explanation of the F1 Cross• The reappearance indicated that at some point

the allele for shortness had been separated from the allele for tallness

• Mendel suggested that the alleles for tallness and shortness in the F1 plants were segregated from each other during the formation of sex cells or gametes

• When each F1 plant flowers, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes – those with the allele for tallness and those with the allele for shortness

11-2 Probability and Punnett Squares

• Mendel kept obtaining similar results, he soon realized that the principals of probability could be used to explain the results of genetic crosses

Probability• The likelihood that a particular

event will occur

• The way in which alleles segregate is random like a coin flip

Punnett Square Vocab

If you do not know the following vocabulary

words you will fail miserably

Punnett Square• Diagram used to determine

genetic crosses

Homozygous• Organisms that have 2 identicle

alleles for a trait

Ex.) TT , tt

Heterozygous• Have two different alleles for a

trait

Ex.) Tt

Phenotype• Physical characteristics – (words)

Ex.) tall

Genotype• Genetic make-up - (letters)

Ex.) Tt, TT, tt

11-3 Exploring Mendelian Genetics

• Mendel wondered if alleles segregate during the formation of gametes independently

• Does the segregation of one pair of alleles affect the segregation of another pair of alleles?

• For example, does the gene that determines whether round or wrinkled in shape have anything to do with the gene for color?

• Must a round seed also be yellow?

All heterozygous 9:3:3:1 Ratio

Independent Assortment• Genes that segregate

independently do not influence each others inheritance

A Summary of Mendel’s Principles

• The inheritance of biological characteristics is determined by individual units known as _______________. In organisms that reproduce sexually, _______________ are passed from parents to offspring

Genes

Genes

A Summary of Mendel’s Principles

• In cases in which 2 or more forms of a gene are present, some forms of the gene may be _______________________ or ___________________________

• In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These genes are segregated from each other when gametes are formed

• The alleles for different genes usually segregate independently of one another

dominantrecessive

Incomplete Dominance• When one allele is not dominant

over another

• Four o’clock flowers

• The heterozygous phenotype is somewhat in-between the two homozygous phenotypes

Codominance• When both alleles contribute to

the phenotype of an organism

Ex.) Speckled Chickens

Multiple Alleles• When more than two possible

alleles exist in a population

Ex.) blood type

• IA

• IB

• i

Dominant

Recessive

Human Blood TypesPhenotype Genotype

A

B

AB

O

IAIA or IAi

IBIB or IBi

IAIB

ii

Polygenic Traits• Traits controlled by two or more

genes

Ex.) eye color, skin color

Genetics and the Environment• The characteristics of any organism,

is not only determined by the genes it inherits

• Characteristics are determined by interactions between genes and the environment

• Ex.) genes may affect a plants height but the same characteristic is influenced by climate, soil conditions and availability of water

Do Now• Human hair is inherited by

incomplete dominance. Human hair may be curly (CC) or straight (cc). The heterozygous genotype (Cc) produces wavy hair. Show a cross between two parents with wavy hair

Do Now• A man is suing his wife on grounds of

infidelity. The man claims that the child is blood type O and therefore must be fathered by someone else. Can he use this evidence in court if he and his wife both have heterozygous B genotypes?

• Show the cross of the two parents

11 – 4 Meiosis

Objectives• What happens during the events

of meiosis?

• What is the difference between mitosis and meiosis?

• Gregor Mendel did not know where the genes he had discovered were located in the cell

• Genes are located on ______________________ in the cell ______________

Meiosis

chromosomesnucleus

Mendel’s principles of genetics require at least 2 things

1. Each organism must inherit… a single copy of every gene from each of its parents

2. When an organism produces its own gametes… these two sets of genes must be separated from each other so that each gamete contains just one set of genes

Chromosome NumberEx.) fruit fly 8 chromosomes

• 4 from mom, 4 from dad

Ex.) Humans 46 chromosomes

• 23 from mom, 23 from dad

Homologous• Chromosomes that each have a

corresponding chromosome from the opposite sex parent

Diploid• A cell that contains both sets

of homologous chromosomes (2N)–Body cells

Haploid• A cell that contains only a single

set of chromosomes (1N)–Sex cells (gametes)

Meiosis• A process of reduction division

in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell–Makes sex cells

Meiosis usually involves 2 divisions

• Meiosis I

• Meiosis II

Meiosis I• prior to meiosis I, each

chromosome is replicated

• The cells then begin to divide similar to mitosis

Prophase I• Each chromosome pairs with its

corresponding homologous chromosome to form a structure called a _____________________ - has 4 chromatids

Tetrad

Crossing over• When chromosomes exchange

portions of their chromatids and results in the exchange of alleles

Crossing over• Leads to new combinations of alleles

• The homologous chromosomes separate, and 2 new cells are formed

• Although each cell now has 4 chromatids something is different. Because each pair of homologous chromosomes was separated, neither of the daughter cells has two complete sets of chromosomes that it would have in a diploid cell

• The two sets have been shuffled

Meiosis II• The two cells produced by

meiosis I now enter a second meiotic division

• Unlike the 1st division, no chromosomes are replicated

• Each cell’s chromosomes has 2 chromatids

Metaphase II• 2 chromosomes line up in the

center of each cell

Anaphase II• The paired chromatids separate

Telophase II• Forms 4 daughter cells each with

2 chromatids

• These 4 daughter cells are now haploid (N) – just 2 chromosomes each

Gamete Formation• In male animals, the haploid

gametes produced by meiosis are called sperm

• In some plants they are called pollen

Spermatogenesis

Gamete Formation• In females, generally only one of the

cells produced by meiosis is involved in reproduction

• This female gamete is called an egg• The other 3 cells that do not receive

as much cytoplasm as the egg are called polar bodies

oogenisis

Comparing Mitosis and Meiosis

• Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells

Comparing Mitosis and Meiosis

46

46 46

46

23 23 23 23

Mitosis Meiosis

11-5 Linkage and Gene Maps

Gene Linkage• When genes are located on the

same chromosome they are inherited together (Linkage)

• It’s the chromosomes that assort independently not individual genes

• When genes are formed on the same chromosome, this does not mean that they are linked forever

• Crossing over during meiosis sometimes separates genes that had been on the same chromosome onto homologous chromosomes

• Cross over events occasionally separate and exchange linked genes and produce new combinations of alleles

Q: Why is this good?A: Generates genetic diversity

Gene Maps

• 1911 Alfred Sturtevant

• hypothesized that the further apart genes were, the more likely they were to be separated by a crossover in meiosis

• the rate at which linked genes were separated and recombined could then be used to produce a “map” of distances between genes

Gene map• Shows the location of each gene