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