chapter 11- genetics meiosis principles of genetics require: -each organism inherits a single copy...

Chapter 11- Genetics

Meiosis

Principles of genetics require:

-each organism inherits a single copy of every gene from each parent

-during gamete formation, two sets of genes must separate, so each gamete contains just one set of genes


Chromosome Number

Fruit fly example:

8 chromosomes total

4 came from male parent

4 came from female parent

Two sets are homologous


Homologous chromosomes- paired chromosomes having genes for the same trait located at the same place on the chromosome

Diploid- having two of each kind of chromosome (2n); normal body cells

Diploid cells have two complete sets of chromosomes/two complete set of genes


Gametes of sexually reproducing organisms have only a single set of chromosomes

Haploid- having one of each kind of chromosome (n)

Fruit fly gametes would be n = 4


Phases of Meiosis

How haploid cells (gametes) produced from diploid cells (somatic cells)

Meiosis- cell division that results in a gamete containing half the number (n) of chromosomes of its parent (2n)

Involves two rounds of division:

-Meiosis I

-Meiosis II


Phases of Meiosis

Meiosis I

Similar to mitosis

Four phases the same: prophase I, metaphase I, anaphase I, and telophase I

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html


Differences-

Prophase I:

Pairing of homologous chromosomes to form tetrad (4 chromatids)

Crossing over- exchange of genetic material between non-sister chromatids of homologous chromosomes

Shuffles genes like a deck of cards


Meiosis II

Second round of division (reduction)

NO chromosome replication

Phases just like meiosis I:

Prophase II, metaphase II, anaphase II, telophase II

At end of division cycle, each cell contains haploid # of chromosomes (n)


Gamete Formation

In male animals, gametes produced through meiosis called sperm

In some plants, pollen contains sperm cells

In female animals, only one cell produced is made into a gamete

Cytokinesis is uneven: produces one egg and 3 polar bodies


Comparing Mitosis & Meiosis

Sexual reproductionAsexual reproduction

Formation of gametesMulticellular organisms: growth & repair

4 genetically different haploid cells

2 identical diploid cells

MeiosisMitosis


The Work of Gregor Mendel

Genetics- scientific study of heredity

Gregor Mendel- Austrian monk who experimented using pea plants

Determined rules of inheritance

Fertilization- joining of male & female gametes to form single cell


Pea plants normally self-pollinating

Mendel used stock of true-breeding plants

(produced offspring identical to themselves)

Cross-pollinated pea plants with different traits to study resulting offspring


Genes & Dominance

Studied 7 different pea plant traits

Trait- a characteristic that can be passed on to offspring

For each trait, studied a contrasting character:

Ex. Seed color- green or yellow


Original pair of plants = P generation

Offspring = F1 generation

Hybrid- offspring of parents that have different forms of a trait

Results?

Every offspring had a character of only one of the parents


Two conclusions:

• Inheritance determined by factors passed from one generation to the next

Gene- the functional units of inheritance

Each trait controlled by one gene with two contrasting forms

Alleles- alternate forms of a gene


Second conclusion called the principle of dominance:

• Some alleles are dominant and others are recessive

Dominant- an allele of a gene that is always expressed (T)

Recessive- an allele of a gene that is expressed only when the dominant allele is not present (t)


Segregation

What had happened to recessive alleles (traits) in plants?

Mendel allowed F1 plants to self-pollinate

Produced F2 generation which showed recessive trait in 1 out of 4 plants (3:1 ratio)


Dominant allele masked recessive trait in F1 generation

Reappearance in F2 generation showed that at some point alleles became separated

Mendel suggested that they segregated from one another during gamete formation


Law of Segregation- states that every individual has two alleles of every gene;

when gametes are produced, each gamete receives one of these alleles


Probability & Punnett Squares

Every time Mendel repeated a particular cross, he obtained similar results

¾ plants showed dominant trait; ¼ showed recessive trait

Mendel realized that probability could be used to explain results


Genetics & Probability

Probability- the likelihood that a particular event will occur

Two possible outcomes for a coin flip:

heads or tails

Probability of each occurring is equal

Chance of heads is 1 in 2, or 50%


Chance of heads coming up three in a row?

Each flip is an independent event:

Probability of three heads in a row-

½ x ½ x ½ = 1/8

Way alleles segregate is random, like a coin flip

• Principles of probability can be used to predict outcomes of genetic crosses


Punnett Squares

Gene combinations from a given cross can be determined using a Punnett square

Letters in Punnett square represent alleles

Gametes from each parent are shown along one side and the top of the square

Used to predict and compare genetic variations resulting from a cross


Homozygous- if an organisms two alleles for a trait are the same

Can be homozygous dominant (TT) or homozygous recessive (tt)

Heterozygous- when the two alleles for the same trait are different (Tt)

Homozygous = true-breeding for trait

Heterozygous = hybrid for trait


Phenotype- the physical expression of the genes; how the trait looks

Ex.-

Tall; short; yellow seeds; green seeds

Genotype- allele combination that an organism contains

Ex.-

TT; Tt; tt


Probability & Segregation

Probability predicts that if the alleles segregate, then a 3:1 phenotypic ratio should be seen

Each cross showed a 3:1 ratio

Mendel’s law of segregation proven correct


Since probabilities predict averages:

the larger the number of individuals, the closer the data comes to the probability


Exploring Mendelian Genetics

Mendel further investigated:

Alleles segregate-

but do alleles package together?

or are they independent of one another?


Independent Assortment

Mendel followed two different genes through two generations

Crossed purebred plants for seed color and seed shape

Round, yellow seeds (RRYY)

with

Wrinkled, green seeds (rryy)


Only provided hybrid plants needed for next cross

All F1 plants had genotype RrYy

F2 generation showed 209 plants that had phenotypes not found in the parents

So…

Alleles for different traits segregate independently of one another


Summary of Mendel’s Principles

• Inheritance is determined by individual units called genes.

Genes passed from parents to offspring.

• When two or more forms (alleles) of a gene exist, some forms may be dominant, others recessive.


• In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent.

These copies are segregated from one another during meiosis (gamete formation)

• Alleles for different genes usually segregate independently of one another


Complex patterns of Inheritance

Important exceptions to Mendel’s principles

Some alleles are neither dominant nor recessive

Many traits controlled by multiple alleles, or multiple genes


Incomplete dominance

Appearance of a third phenotype

Due to one allele being not completely dominant over another

3rd phenotype somewhere in between the two homozygous phenotypes


CodominanceExpression of both

alleles (ex. White bull

crossed with a red cow yields a roan calf)

Both alleles contribute to the phenotype


Multiple phenotypes from multiple alleles• More common for multiple alleles to

control a trait in a population• Only two alleles of a gene can exist within

the cell • Multiple alleles for a single gene can be

found within a populationExamples:Blood type- A, B, oRabbit coat color- C, cch, ch, c


Polygenic Inheritance

• Traits that are determined through expression of two or more genes

• Polygenic traits show wide range of phenotypes

• Ex- eye color, skin color, height


Genetics & the Environment

Characteristics of any organism not solely determined by genes

Determined by interaction between genes and environment

Genes provide plan, how plan goes also depends on environment

chapter 11- genetics meiosis principles of genetics require: -each organism inherits a single copy...

Documents

homologous slide

chromosomes n slide

yellow slide

genetics phases of meiosis

single cell slide

genetics pea plants

genetics gamete formation

genetics genes dominance