Mendel and Meiosis

Chapter 11

Mendel’s Laws of Heredity Heredity – passing on traits from

parents to offspring Gametes – sex cells; they have

a haploid number of chromosomes; they have only ½ as many chromosomes as normal cells or one copy of each gene

Fertilization – when a male gamete unites with a female gamete

Zygote – fertilized cell Genes are chemical factors that

determine traits Alleles – different forms of a

gene

Mendel’s experiments Pea plants are self-pollinating; male

and female parts in the same flower Only studied one trait at a time and

subjected his data to statistical analysis

First cross was between two purebreds or true breeding plants (tall x short)

All offspring were tall

Mendel’s experiments (cont) Genetically speaking, the result

was a hybrid with one allele for tall and one for short

Then, Mendel allowed the hybrids to self-pollinate (hybrid x hydbrid)

Second generation offspring were ¾ tall and ¼ short

Rule of Unit Factors

Each organism has two factors/alleles for each trait; the two alleles are located on different copies of a chromosome, one from each parent

Rule of Dominance

In a hybrid where only one trait can be observed, that trait is said to be dominant over the trait that is not visible; the other trait is said to be recessive

Law of Segregation

In sexually reproducing organisms, offspring have one gene from each parent, these are segregated in gamete (sex cell) formation

Law of Independent Assortment

Genes for different traits are inherited independently of each other

Phenotypes and Genotypes Phenotype is the physical appearance

of a trait in an organism Genotype is the gene/allele

combination of an organism Homozygous is an organism that has

two of the same alleles for each trait Heterozygous is an organism that has

two different alleles for each trait

Crosses

Monohybrid cross = Mendel’s first crosses; studying one trait and its inheritance at a time

Dihybrid cross studies the relationship between the inheritance of 2 traits

Punnett Squares Monohybrid cross (one trait) is a two by

two grid One parent’s genotype is listed at the

top; one allele per column The other parent’s genotype is listed

along the side; one allele per row Squares are filled in with one allele

from one parent (top) and one from the other (side)

Dihybrid cross (two traits) is a four by four grid

Very similar to the monohybrid cross, but you must account for all possible gene combinations from parents

probability chance that you will get one

result over another Punnett squares are a way to

determine probability the more data you have, the

closer to the predicted probability your results should be

Meiosis

Genes and chromosomes Diploid – 2n - cell/organism

that contains two of each type of chromosome (one copy from each parent)

Haploid – n - cell (sex cell) that contains only one copy of each type of chromosome

Homologous chromosomes – two chromosomes of the same type or a pair of similar chromosomes in a diploid cell

Meiosis – process of cell division that reduces the number of chromosomes in a cell from diploid to haploid; used for reproduction sperm – male gametes egg – female gametes

Meiosis I – similar to mitosisEXCEPT: In Prophase I, homologous

chromosomes come together as tetrads

during metaphase I, tetrads line up along the center of the spindle

the tetrad is pulled apart during anaphase I, so the homologs are now separate from each other in two different cells

when tetrad are formed, crossing over may happen; this basically means that homologous chromosomes could “trade ends

Meiosis II no chromosome replication before

meiosis II chromosomes line up along center

of spindle in metaphase II sister chromatids are pulled apart

in anaphase II result of both divisions together is

4 haploid cells

Gamete FormationMeiosis produces 4 genetically

different haploid cells, whereas mitosis produces 2 genetically identical cells.

In males, 4 equal-sized gametes are produced.

Gamete FormationIn females, only 1 large egg cell

is produced, along with 3 cells called polar bodies that are not involved in reproduction.

Genetic Recombination Crossing over just refers to “trading

ends” between homologous chromosomes during the tetrad grouping in prophase I and metaphase I

Also caused by the law of independent assortment; different chromosomes will be segregated independently of one another

Nondisjunction

when chromosomes fail to separate properly during meiosis

Monosomy – lacking a chromosome Trisomy – extra chromosome Tetrasomy – 2 extra chromosomes

Polyploidy

Organisms have more than 1 set of chromosomes

Rare in animals; usually results in death or, in humans, mental retardation

Plants have a higher occurance rate; can create a “better” or healthier plant, commercially or biologically

Gene Linkage and Maps

Genes on the same chromosome are said to be linked

If they are far apart on the chromosome, crossing over is more frequent

genes maps are charts of chromosomes with gene locations on them (ex. Human Genome Project)