Meiosis to Mendel

Chapter 9

Reproduction

• Asexual reproduction – mitosis– Produces clones – genetically identical

individuals

• What would happen if the environment changed?

WHY SEX ?• Sexual reproduction combines the DNA

from two different individuals

• A gene is a section of a chromosome that carries instructions for a specific trait (protein)

• The greater the number of different combinations of genes the more variation among individuals, and the greater the chance of survival of the species.

Sex cell formation

• Sexually reproducing organisms need to produce specialized reproductive cells or gametes.

• Produced from germ cells in organs called gonads

• In females ovaries produce eggs or ova

• In males testes produce spermatozoa

Somatic (body) cells contain pairs of chromosomes or the diploid (2n) number of chromosomes.

Homologous pairs – each member is a homolog

Gametes contain only one member of each pair or the haploid (n) number of chromosomes

• When a sperm and egg combine in the process of fertilization, or syngamy, a new diploid cell or zygote is formed.

• To form haploid gametes, there needs to be a process other than mitosis – this is called meiosis

Meiosis• This is a two part process : meiosis I and

meiosis II

• However, the DNA is only replicated once

• Meiosis I and II both use the same four stages of mitosis: prophase, metaphase, anaphase and telophase

During prophase I the homologous chromosomes pair up in synapsis. This is the longest phase of meiosis. Crossing over may occur further increasing genetic variation.

• In metaphase I the tetrads migrate to the center of the cell.

• In Anaphase I the centromeres do not break and one member of each homologous pair (2 sister chromatids) move to opposite ends of the cell

• Which homolog goes to which end of the cell occurs at random.

• Telophase occurs as in mitosis.

Meiosis II• There is no replication of chromosomes

between telophase I and prophase II

• Meiosis II proceeds just like mitosis – during anaphase the centromeres break and the two sister chromatids go to opposite poles.

Cytokinesis• Varies by which type of cell is being made

• If we are producing sperm, each of the four cells produced by meiosis II become sperm.

• If we are making ova, cytokinesis is uneven and one cell takes nearly all the cytoplasm, leaving the other cell merely a package of discarded DNA called a polar body.

• In humans, the cell again divides unevenly, so at the end of meiosis II we have formed one ovum (egg) and three polar bodies. The polar bodies disintegrate.

• The average woman produces one ovum every 28 days

• Males produce 300 million sperm/ day

• If less than 20 million / ml, a man is considered infertile.

• Fertilization is a group effort, but only one sperm penetrates the ovum.

• Sometimes things go wrong.• In anaphase I the separation of

homologous chromosomes is called disjunction.

• When they do not separate it is called nondisjunction and the resulting gametes contain one too many or one too few chromosomes.

• Fertilization results in a zygote with 45 or 47 chromosomes. This is an aneuploid (vs. euploid) number of chromosomes

• Three copies of a chromosome is called trisomy – Down syndrome is trisomy 21

• A zygote with one too few chromosomes does not usually develop.

• Extra copies of the sex chromosomes (vs. the autosomes) do not cause as much of a problem

• XX is female in humans (male in birds)

• XY is male in humans

• XXY , XYY

Genetics• A gene is a section of DNA that codes for :

– Proteins – for structures such as muscles - or for enzymes

– Regulatory genes – areas of DNA that regulate the expression of structural genes

• Each cell of an organism that reproduces sexually has two copies of each chromosome, and therefore has two copies of every gene – one on each member of each pair of chromosomes(exception is the Y chromosome, which is smaller than the X).

• The two versions of each gene are called alleles. Alleles may be the same or different, depending on the traits of the parents.

• All the genes that are contained on all the chromosomes of an individual make up that individuals Genotype or Genome.

• Genotype can also be used to refer to the genes for a particular trait.

• Not all genes are expressed.

• Those traits that are expressed: can be seen (physical traits) or measured (chemical traits) are the individual’s phenotype.

• The phenotype is influenced by both the genotype and the environment.

Transmission Genetics• How traits are passed down from

generation to generation.

• Transmission of genes and the phenotypes which come from those genes

• The phenotype determines how the individual interacts with the world, and it is the phenotype that is subject to natural selection.

• If an organism has two copies of the same allele is it said to be homozygous.

– True breeding

• If an organism has different alleles of the same gene it is said to be heterozygous.

If an allele is dominant, it is expressed whenever that allele is present.

A recessive trait is only expressed when the trait is homozygous.

Dominant traits are written with capital letters.

Recessive traits are written with small letters.

P = purple pigment (Purple flowers)

p = no pigment (white flowers)

PP = ?

Pp = ?

pp = ?

PP and Pp = purple flowers

pp = white flowers

• Breeding two genetically distinct organisms is called cross breeding or crossing.

• The offspring of such crosses are called hybrids.

• The parents are called the parental or P generation

• The offspring of these parents are called the F1 generation (first filial)

• What would we get if we crossed a homozygous purple flower (PP) with a homozygous white flower (pp)?

• To find out, we can use a Punnett square – named after Reginald Punnett

P P

p Pp Pp

p Pp Pp

Parent 2

Parent 1

Gregor Mendel• 1856 -1863• monk, Czech Republic• Studied 7 traits in pea plants, Pisum sativum

– Established basic rules of transmission genetics• Good science, but ignored for >30 years

• Why peas?– Many varieties with contrasting traits– Self-pollinating, with true-breeding varieties– easy to snip parts to cross pollinate– Need little space, produce lots of offspring

His experiments would not have worked out except:

1) He chose traits that were all dominant or recessive

2) He chose traits that were all located on different chromosomes (pea plants

have 7 chromosomes)

Pretty amazing since he had no idea how these traits were passed on – he called them “unit factors”

• What would happen if we crossed members of the F1 generation?

P p

P

p

Genotypes: 1: 2 : 1 PP : Pp : pp

What if we had a plant that had a dominant characteristic and wanted to know if it was homozygous or heterozygous ?

We could do a back cross or test cross – breed the individual with a homozygous recessive individual.

Genotypic ratio 1:1 Pp : pp

If the purple plant was homozygous, the F1 generation would all be purple - Pp

But, not all traits show simple dominant-recessive relationships. There is also partial dominance where both traits are expressed.

Some traits show incomplete dominance.Snap dragons have genes for red flowers (R1)

and white flowers (R2). A heterozygous flower (R1R2) would be

Pink!This type of trait gave early scientists the idea that traits blended in offspring of different individuals.

Notice that both traits are given capital letters, and the F2 generation shows a 1:2:1 ratio of phenotypes as well as genotypes.

Other traits show Codominance where both alleles are equally expressed.

Blood types: A B O blood groups

Isoantigens – particular proteins on the cell membrane. IA, IB and i (O is recessive, and this is a case of multiple alleles)

IAIA = Type A IBIB = Type B

IAi = Type A IBi = Type B

IAIB = Type AB ii = Type O

Mendel’s Crosses Showed:• Principle of segregation: each sexually

reproducing organism has two genes for each characteristic, and these two genes segregate or separate during the production of gametes.

• Principle of independent assortment: traits which lie on different chromosomes are passed on independently of each other.

With these rules in mind, we can cross individuals that have two different traits.

Dihybrid (vs. monohybrid) cross.

We can cross peas that have green pods (G) which are inflated (I) with peas that have yellow pods (g) which are constricted (i).

GGII X ggii = GgIi

parents F1

The only “trick” to a dihybrid cross is setting up the Punnett square.

GgIi →

GI, Gi, gI, gi

Like making a snack – take one of each

GI Gi gI gi

GI GGII GGIi GgII GgIi

Gi GGIi GGii GgIi Ggii

gI GgII GgIi ggII ggIi

gi GgIi Ggii ggIi ggii

Genotypes :

1 GGII : 2 GgII : 2 GGIi : 4 GgIi

1 GGii : 2 Ggii 1 ggII : 2 ggIi

1 ggii

Phenotypes: 9 Green inflated

3 Green constricted

3 Yellow inflated

1 Yellow constricted

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Sex-Linked traits

• Some genes carried on the X chromosome are missing from the Y chromosome.

• These traits show up in different ratios in males and females and are called sex-linked traits

• Males are said to be hemizygous for these traits since they can only have one gene and a recessive gene will always be expressed.

Color blindness is carried on the X chromosome (X’) Normal color vision (X)

X Y The females all have

X XX XY normal color vision, but

half the males are color

X’ XX’ X’Y blind.

X’ Y Here half the females

X XX’ XY and half the males are

X’ X’X’ X’Y color blind.

• In 1902 Mendel’s work was rediscovered and William Bateson and Walter Sutton realized that the behavior of chromosomes during meiosis explains Mendel’s principles of segregation and independent assortment.

• Walter Sutton published his own 6 rules of inheritance.

What happens when two traits are located on the same chromosome?They tend to be passed on together – this is

called genetic linkage.

Can these two traits be inherited separately?

The likelihood that two genes on the same chromosome will be inherited separately depends on the distance between them.

A map unit is defined as the distance between two genes that produces a 1 percent recombination in gametes.

What is the greatest possible distance (in map units) between two genes?

50 map units.

If the genes were on separate chromosomes they would end up in different gametes 50 % of the time.