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Genetics
Unit 7
General Biology
Chromosome Number
• The Chromosomal Theory
of Inheritance – genes are
located in specific positions
on chromosomes.
• Homologous Chromosomes
– chromosomes come in
pairs, one from the male
parent and one from the
female parent.
Gene Map
Chromosome Number
• Diploid – a cell that contains both sets of
homologous chromosomes. (2N)
• Diploid cells contain two complete sets of
chromosomes and two complete sets of genes (one set
from each parent).
• Haploid – a cell only containing one set of
chromosomes. (N)
Meiosis
• Meiosis – a process of reduction division in which
the number of chromosomes is cut in half through
separation of homologous chromosomes in a diploid
cell.
• Meiosis takes place in two distinct divisions: Meiosis I
and Meiosis II.
Meiosis
• Interphase – cells undergo DNA replication, forming
duplicate chromosomes during the S phase.
• Meiosis I
• Prophase I – each chromosome pairs with its
corresponding homologous chromosome to form a
tetrad. Crossing over occurs in prophase I.
• Metaphase I – chromosomes line up in the middle of
the cell and attach to spindle fibers.
• Anaphase I – spindle fibers pull chromosomes toward
opposite ends of the cell.
• Telophase I and Cytokinesis – nuclear membrane
reforms and the cell divides into two cells.
Meiosis I
• Crossing Over – in prophase I,
homologous chromosomes
exchange portions of their
chromatids.
• This produces new combination of
alleles and allows for more genetic
variation.
Meiosis I
Meiosis
• Meiosis II
• Prophase II – meiosis I resulted in two haploid
daughter cells with half the number of chromosomes as
the original cell.
• Metaphase II – the chromosomes line up in the middle
of the cell.
• Anaphase II – sister chromatids are separated and
move toward opposite ends of the cell.
• Telophase II and Cytokinesis – nuclear membranes
form and meiosis II results in four haploid daughter
cells.
Meiosis II
Gamete Formation
• In male animals, meiosis results in four equal-sized
gametes called sperm.
Gamete Formation
• In many female animals, only one egg results from
meiosis. The other three cells, called polar bodies,
are usually not involved in reproduction.
Comparing Mitosis and
Meiosis
• Mitosis results in the production of two genetically
identical diploid cells, whereas meiosis produces
four genetically different haploid cells.
• The physical processes that occur during meiosis II
is identical to the physical processes that occur
during mitosis.
Gregor Mendel
• Gregor Mendel is known as the father of genetics. In
1866, he published his findings on the method and
mathematics of inheritance in garden pea plants.
• Pea plants reproduce by self-fertilization. Selffertilization occurs when a male gamete within a flower
combines with a female gamete in the same flower.
• Mendel discovered that pea plants could be easily crosspollinated.
• As Mendel bred his pea plants, he analyzed his results
using mathematics to form hypotheses concerning how
traits were inherited.
Flower Anatomy
Genetics
• The passing of traits
from one generation to
the next is called
inheritance, or
heredity.
• Genetics is the study of
heredity.
The Inheritance of Traits
• Mendel noticed that certain varieties of garden pea
plants produced specific forms of a trait, generation
after generation (like yellow and green seeds).
• To begin to understand how the traits were inherited,
he used cross-pollination:
• Transferring male gametes from a true-breeding greenseed pea plant to the female organ of a flower from a
true-breeding yellow-seed pea plant.
• He called this parent generation, the P generation.
The Inheritance of Traits
• The offspring of the P cross, called the F1
generation, all had yellow seeds…
• Why didn’t any of have green seeds if one of the
parents had green seeds???
• Mendel allowed the F1 generation to self-fertilize
and the offspring of the F2 generation had mostly
yellow seeds, but some green seeds too…
• How did the green seeds reappear in the F2
generation???
Conclusions From The
Experiment
• There must be two forms of the seed-color trait in
the pea plants (yellow and green)
• These are called alleles, or difference forms of a
single gene/trait
• The gene/trait: seed color
• Alleles: green or yellow
• Based on his observations, he decided that some
alleles must be dominant over others. We called the
allele that gets masked recessive.
Representing Alleles
• Alleles that are dominant are represented with
capital letters.
• Alleles that are recessive are represented by the same
letter as the dominant allele for the trait, just lowercase.
• For example: If yellow seeds are dominant over
green seeds.
• The dominant allele, yellow seeds: Y
• The recessive allele, green seeds: y
You try…
• In pea plants, if round seeds are dominant over
wrinkled seeds
• The dominant allele, round seeds:
• The recessive allele, wrinkled seeds:
• In pea plants, if tall stems are dominant over short
stems
• The dominant allele, tall stems:
• The recessive allele, short stems:
Homozygous and
Heterozygous
• Remember that each offspring has an allele for each
trait from both parents.
• If both alleles are the same, we say that the offspring
is homozygous for that trait.
• YY (homozygous dominant)
• yy (homozygous recessive)
• If the two alleles are different, we say that the
offspring is heterozygous for that trait.
• Yy
Genotype and Phenotype
• The organisms allele pairs (YY, Yy, or yy) is called
its genotype.
• The observed characteristic or outward expression
(yellow or green) of an allele pair is called the
phenotype.
The Laws
• During Mendel’s study of heredity in pea plants, he
was able to develop two laws:
1. Law of Segregation
2. Law of Independent Assortment
Law of Segregation
• The law of segregation states that two alleles for
each trait separate during the formation of gametes
(meiosis).
• During fertilization, two alleles for that trait unite.
Monohybrid Cross
• A cross that involves
hybrids for a single trait is
called a monohybrid cross.
• This occurred during the
self-fertilization of Mendel’s
F1 generation.
• Yy x Yy
Dihybrid Cross
• The simultaneous inheritance of two or more traits
in the same plant is a dihybrid cross.
• Dihybrids are heterozygous for both traits
• YyRr x YyRr
Law of Independent
Assortment
• The law of independent assortment states that a
random distribution of alleles occurs during
metaphase I of meiosis as chromosomes align down
the center of the cell.
• Therefore, the genes of one trait do not influence the
genes of another trait.
Fig. 13-11-1
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Fig. 13-11-2
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Fig. 13-11-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
Dr. Reginald Punnett
• In the early 1900’s, he developed what is known as the
Punnett square to predict the possible offspring of a
cross between two known genotypes.
• Punnett squares can be used to determine possible
genotypes and phenotypes of the cross.
• These can be represented as ratios:
• Genotypic ratio
• Phenotypic ratio
Using a Punnett square
1. Create a box with 4 squares.
2. Identify the alleles for the
trait/gene (T and t).
3. Identify the genotypes of the
individuals being crossed.
4. Place the alleles for the
genotypes in the appropriate
places around the box.
5. Fill in the box by carrying the
letter across and down.
Single Factor Cross
Two Factor Cross
Complex Inheritance Patterns
• Some alleles are neither dominant nor recessive, and
many traits are controlled by multiple alleles or
genes.
• These types of inheritance patterns are called
complex inheritance patters
Incomplete Dominance
• Incomplete Dominance –
one allele is not
completely dominant
over another.
• In incomplete dominance,
the heterozygous phenotype
is somewhere in-between the
two homozygous
phenotypes.
Codominance
• Codominance – both alleles contribute to the
phenotype.
• Example: AB blood type
Multiple Alleles
• Multiple Alleles – genes having
more than two alleles.
• This does not mean that an
individual can have more than two
alleles, but it means that more than
two possible alleles exist in a
population for a given trait.
• Example: human blood type (A, B,
AB, O)
Multiple Alleles
• Human Blood Groups
• The ABO blood group has three alleles IA, IB, and i.
• Alleles IA and IB are codominant. These alleles
produce molecules known as antigens on the surface of
red blood cells.
• The i allele is recessive to both IA and IB and produces
no antigen.
Multiple Alleles
• Human Blood Groups
Polygenic Traits
• Polygenic Traits – controlled by two or more genes.
• Example: skin color of humans—controlled by more
than 4 different genes
Applying Mendel’s Principles
• Mendel’s principles don’t apply only to plants, but other
organisms and humans too.
• In the early 1900s, Thomas Hunt Morgan found a model
organism to advance the study of genetics: the common
fruit fly.
• Fruit flies were an ideal organism for several reasons:
• They reproduced quickly and had many offspring
• Morgan and other biologists learned that Mendel’s
principles applied not to just pea plants, but all life—since
DNA is universal and contains genetic information.
Genetics and the Environment
• The characteristics or phenotypes of any organisms
are not determined solely by the genes it inherits, but
by the interaction between genes and the
environment.
• Example: Genes may affect a sunflowers height and the
color of its flowers, but these same characteristics are
also influenced by climate, soil conditions, and
availability of water.
Karyotype Studies
• The study of genetic material doesn’t involve genes
alone.
• Scientists also study whole chromosomes by using
images of chromosomes taken during mitosis.
• A stain is used to identify or mark identical places
on homologous chromosomes.
• The pairs of homologous chromosomes are
arranged in decreasing size to produce a diagram
called a karyotype.
Karyotype (male or female?)
Karyotype (male or female?)
Pedigree Charts
• A pedigree is a diagram that traces the inheritance of a
particular trait through several generations.
• A pedigree uses symbols to illustrate the inheritance:
•
•
•
•
•
Males are represented by squares
Females are represented by circles
One who expresses a trait is dark or filled
One who doesn’t express the trait in unfilled
One who is a carrier is half shaded (heterozygous)—only done
in recessive disorders
Pedigree Charts
• A horizontal line between two symbols shows that
these individuals are the parents of the offspring
listed below them.
• Offspring are listed below them, oldest on the left to
youngest on the right.
• A numbering system is used in which Roman
numerals represent generations and Arabic numbers
are used to describe birth order.
Pedigree
Analyzing Pedigrees
• A pedigree shows an individual’s phenotype
• You can analyze a pedigree to infer genotypes and
whether the trait that is being inherited is a recessive
or dominant genetic disorder
• Pedigrees are useful if good records have been kept
within families. It allows genetic disorders in future
offspring to be predicted.
Dominant or Recessive
Disorder?
• View the pedigree below. Is this disorder dominant
or recessive?
Dominant or Recessive
Disorder?
• View the pedigree below. Is this disorder dominant
or recessive?
Genetic Disorders: Recessive
Disorders
• Many disorders seen in humans are caused by
genetics.
• A recessive disorder is expressed when the individual
is homozygous recessive for the trait.
• An individual that is heterozygous for a recessive
disorder, and therefore doesn’t express it, is called a
carrier.
Recessive Disorders
Genetic Disorders: Dominant
Disorders
• Not all disorders are caused by recessive inheritance.
Some are cause by dominant alleles.
• Dominant disorders are not present in individuals
that are homozygous recessive for the trait.
Understanding Genetic Disorders
through Genetics Counseling
Meiosis and Nondisjuction
• Recall, that meiosis is the process used to form
gametes (Diploid cell  haploid cells)
• During meiosis I, homologous chromosomes are
separated.
• During meiosis II, sister chromatids are separated.
• If homologous chromosomes or sister chromatids
don’t separate properly during meiosis, this is known
as nondisjunction.
Nondisjunction
Down’s Syndrome
• Trisomy 21
• Characteristics of the disorder:
•
•
•
•
Distinctive facial features
Short stature
Heart defects
Mental disability
Sex Determination
• Your gender is inherited based
on your 23rd pair of
chromosomes, called sex
chromosomes.
• 2 types: X and Y
• XX = female
• XY = male
• The other 22 pairs of
chromosomes are called
autosomes.
Sex-Linked Inheritance
• Sex-Linked Genes – genes
located on the sex
chromosomes are said to be
sex-linked.
• Males have just one X
chromosome, thus all X-linked
alleles are expressed in males,
even if they are recessive.
Sex-Linked Inheritance