Chapter 11

Introduction to Genetics

Chromosomes and Cells

Two general types of cells– Somatic cells-body cells that make up the

tissues and organs– Gametes-sex cells (eggs and sperm)

Chromosomes and Cells

Characteristic number of chromosomes in cells of each species’ body cells

Gametes have ½ of that number of chromosomes

Human body cells have 46 chromosomes in every body cell but only 23 chromosomes in each gamete

Chromosomes and Cells

Chromosomes are grouped in pairs (homologous pairs) according to size, shape, and genes they carry

There are 23 pairs of chromosomes in human cells– Pairs 1-22 are autosomes – Pair 23 is sex chromosomes

Chromosomes and Cells

Somatic cells are diploid (having the “normal” number of chromosomes / cell = 46 for human somatic cells)

Gametes are haploid (having ½ the normal number of chromosomes / cell = 23 for human gametes)– Gametes are haploid because the diploid

number is reinstated at fertilization

Karyotype

Meiosis

Germ cells undergo meiosis to produce gametes

Meiosis is a form of nuclear division that divides diploid cells into haploid cells– Reduces the number of chromosomes– Essential for sexual reproduction

Meiosis

Germ cells undergo meiosis to produce gametes

Meiosis is a form of nuclear division that divides diploid cells into haploid cells– Reduces the number of chromosomes– Essential for sexual reproduction

Comparing mitosis and meiosis

MITOSIS– 1 nuclear division– Begin with 1 diploid cell– End w/ 2 diploid cells– Occurs in somatic cells– Daughter cells are identical

MEIOSIS– 2 nuclear divisions– Begin w/1 diploid cell– End w/4 haploid cells– Occurs in gametes or sex cells– Daughter cells are NOT identical

11-4 Meiosis

Involves 2 nuclear divisions

Results in the production of gametes or sex cells (egg and sperm)

Reduction division to reduce the number of chromosomes by half in the gametes (chromosomes return to normal number at fertilization)

Meiosis I

Begin with a diploid cell

Prophase I– Homologous chromosomes pair up to form

TETRADS (4 chromatids per tetrad)– Crossing over occurs: chromatids exchange

genes to create larger genetic variation– Nucleolus and nucleus dissolve– Spindles begin to form

Meiosis I

Metaphase I– Tetrads line up at equator– Each centromere is attached to a spindle

Meiosis I

Anaphase I– Homologous chromosomes separate and

begin moving to opposite poles

Telophase I and cytokinesis– Nuclear membranes form around each set of

chromosomes– Cell separates into two new cells – Each new cell is now HAPLOID

Meiosis II

Begin with 2 haploid cells

Prophase II– Nuclei dissolve– Chromosomes are visible

Metaphase II– Chromosomes line up at the equator of each

cell– Centromeres are attached to spindles

Meiosis II

Anaphase II– Chromatids split in each cell and move to

opposite poles

Telophase II and cytokinesis– Nuclei form around each set of chromosomes– Each cell divides into two new haploid cells

Results of Meiosis

4 haploid daughter cells that are NOT identical

These cells will develop into gametes in a process known as gametogenesis– Males: all 4 cells develop into sperm– Females: only 1 cell receives enough

cytoplasm to become an egg; the other 3 become polar bodies and are reabsorbed

11-1 Gregor Mendel

Father of genetics

Born 1822

Austrian Monk

Attended the University of Vienna

Did all genetic research in the gardens at the monastary

Studied pea plants

Pea plants

Naturally true-breeding (can self-fertilize)

Can manipulate pollination for cross breeding to produce hybrid offspring

Hybrids are offspring from two parents having contrasting characters for a trait

Traits studied on peas

Seed shape

Seed color

Seed coat color

Pod shape

Pod color

Flower position

Plant height

Genes

Genes are chemical factors that control traits

Located on chromosomes

Have alternate forms call ALLELES

Each individual has 2 alleles for each trait (one allele coming from the mother one coming from the father)

Dominant and recessive

Dominant traits– show in all generations– Represented by capital letters

Recessive traits– Absent in first generation but reappear in

second generation– Represented by lower-case letters

11-2 Probability

Probability is the likelihood that an event will occur.

Probabilities are used to predict the outcomes of genetic crosses

To show probabilities of genetic crosses Punnett Squares are used

Punnett Squares

Show all possible gene combinations

Can be used to predict and compare genetic variations that result from a cross

Show genotypes (genetic makeup) using letters

Relate phenotypes (physical characteristics)

Genotypes

Homozygous have two of the same alleles– rr– RR

Heterozygous have two different alleles– Rr– Tt

11-3 Independent assortment

Genes that segregate independently do NOT influence each other’s inheritance

Genes are not inherited together

Leads to large variations in genetics of offspring

Crosses

Monohybrid cross involves one trait

Dihybrid cross involves two traits

P1 generation- true-breeding parents

F1 generation-first generation of hybrid offspring from P1

F2 generation-second generation of hybrid offspring (F1 parents)

Mendel’s principles

Inheritance of characteristics is determined by genes

Genes may have alternate forms called alleles; some dominant, some recessive

In sexually reproducing organisms, adults have 2 copies (alleles) for each gene-one from each parent

Alleles segregate independently

Other forms of inheritance

Incomplete dominance

Codominance

Multiple alleles

Polygenic traits

Incomplete dominance

Neither allele is completely dominant over the other

Heterozygotes will exhibit a “mixing” of traits or an intermediate phenotype

Codominance

Both alleles are equally dominant and contribute equally to the heterozygous phenotype

Multiple alleles

More than 2 alleles involved with the trait of that particular population

Rabbit fur color– C – brown; cch-gray, ch-white w/ brown areas,

c-albino

Polygenic traits

Traits produced by the interaction of several genes

At least 3 genes involved

Example: human skin color or human height

Genetic recombination

Sexual reproduction gives genetic variation in the offspring– Due to independent assortment of

chromosomes in meiosis– Mixing of alleles when gametes fuse in

fertilization– Produces unique combinations of alleles

Genetic recombination

Crossing over– Increases genetic variation– Occurs only in prophase I of meiosis– Chromatids of homologous chromosomes will

exchange (trade) some alleles

Genetic linkage

Genes located close together on a chromosome are usually inherited together because they are “linked”

Genes located far apart or on different chormososmes are inherited independently

Genetic linkage