Transcript Introduction to Genetics

Introduction to Genetics
Chapter 11
Warm Up
• Please complete the pretest that you picked
up as you came in!
The Work of Gregor Mendel
• Genetics- the study of heredity.
• Gregor Mendel- Austrian monk- the father of
genetics- carried out his work on garden peas.
– Pea flowers are naturally self-pollinating, which
means that sperm cells fertilize the egg cells in the
same flower, thus the new flower inherits all of
their characteristics from the single plant that
bore them.
The Work of Gregor Mendel
• True-Breeding- organisms that
produce offspring identical to
themselves if allowed to selfpollinate.
– ie: one stock of seeds would
produce only tall plants, another
only short plants. One stock
would produce yellow seeds,
another only green seeds.
The Work of Gregor Mendel
• Cross-Pollination- produced seeds that had
two different plants as parents.
Genes and Dominance
• Trait- a specific
characteristic that varies
from one individual to
another.
• Each original pair of plants
is called the P (parental)
generation.
• The offspring are called the
F1 (first filial) generation.
Genes and Dominance
• Hybrids- the offspring of crosses with different
traits.
• Genes- chemical factors that determine an
organism’s traits. Genes are passed from
parents to their offspring (one gene from each
parent).
• Alleles- different forms of a gene.
– ie: forms that produce tall vs. short plants or
round vs. wrinkled seeds.
Genes and Dominance
• Principle of Dominance- some alleles are
dominant and others are recessive.
– An organism with a dominant allele for a specific
form of a trait will always exhibit that form of the
trait.
– An organism with a recessive allele for a specific
form of a trait will exhibit that form only when the
dominant allele for the trait is not present.
Segregation
Segregation
• The reappearance of the recessive trait
indicated that at some point the paired alleles
are separated or go through segregation
during the formation of gametes.
• Gametes- sex cells
Exit Slip
• Explain the difference between true-breeding
and hybrid plants.
Warm Up Exercise
• What are dominant and recessive alleles?
How do we represent them?
• What are gametes?
Genetics and Probability
• Probability- the likelihood that a particular
event will occur.
– Remember, past outcomes do not affect future
outcomes.
– The principles of probability can be used to
predict the outcomes of genetic crosses.
Punnett Squares
• Punnett Square- a
diagram that
determines what gene
combinations can
result from a specific
genetic cross.
Punnett Squares
• Homozygous- organisms that have two
identical alleles for a particular trait (TT or tt).
– Homozygous organisms are true-breeding for a
particular trait.
• Heterozygous- organisms that have two
different alleles for the same trait.
• Phenotype- physical characteristics.
• Genotype- genetic makeup.
– Homozygous dominant, heterozygous,
homozygous recessive
Probability
• Probabilities predict averages. Thus, the
larger the number of offspring, the closer the
resulting numbers will be the expected values.
Exit Slip
• An F1 plant that is homozygous for shortness is
crossed with a heterozygous F1 plant. What is
the probability that a seed from the cross will
produce a tall plant? Use a punnett square to
explain your answer.
Warm Up Exercise
-Determine what maternal genotype(s) would most likely yield offspring with such
characteristics.
-Use the genotype that you came up with for the mother to complete a punnett
square.
The Two Factor Cross
The Two Factor Cross
Independent Assortment
• Independent Assortment- genes for different
traits segregate independently, such that the
genes for one trait do not influence another
trait.
Beyond Dominant and Recessive
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
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.
– Ex: 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, it means that more than
two possible alleles exist in a population for a
given trait.
– Ex: human blood type
Hair Color and Multiple Alleles
• FIGURE 11.12
Polygenic Traits
• Polygenic Traits- controlled by two or more
genes.
– Ex: skin color of humans- controlled by more than
four different genes.
Applying Mendel’s Principles
• Mendel’s principles don’t apply only to plants.
• In the early 1900s, Thomas Hunt Morgan found a
model organism to advance the study of genetics,
the common fruit fly, Drosophila melanogaster.
• Fruit flies were an ideal organism for several
reasons:
– They could produce plenty of offspring, and they did
so quickly
• Morgan and other biologists learned that
Mendel’s principles applied not to just pea plants,
but other organisms and humans too.
Genetics and the Environment
• The characteristics of any organism are not
determined solely by the genes it inherits,
but by the interaction between genes and the
environment.
– Ex: 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.
Warm Up Exercise
• In rabbits, black hair is due to a dominant
gene B, and brown to its recessive allele b.
Short hair is due to the dominant gene S and
long hair to its allele s. In a cross between
homozygous black, long hair individual with a
homozygous short, brown hair individual,
what would be the nature of the F1
generation?
Chromosome Number
• The Chromosomal Theory of Inheritancegenes are located in specific positions on
chromosomes.
• Homologous- chromosomes form in pairs, one
from the male parent and one from the
female parent.
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.
• 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. Nucleus breaks down.
• 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 combinations 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.
Exit Slip
• In human cells, 2N = 46. How many
chromosomes would you expect to find in a
sperm cell? In an egg cell? In a white blood
cell?
Gene Linkage
• Thomas Hunt Morgan studied gene linkage on
fruit flies. His conclusions:
– Each chromosome is actually a group of linked
genes.
– Mendel’s principle of independent assortment still
holds true.
• It is the chromosomes that assort
independently, not the individual genes.
Gene Maps
• Crossing-over during meiosis sometimes
separates genes that had been on the same
chromosomes onto homologous
chromosomes.
– Crossover events occasionally separate and
exchange linked genes and produce new
combinations of alleles, which helps generate
genetic diversity.
Gene Maps
• Alfred Sturtevant (student to Morgan)
– The further apart two genes are, the more likely
they are to be separated by a crossover.
– This allowed him to use recombination
frequencies to determine the distances between
genes.
Gene Maps
• Gene Map- shows the relative location of
genes on a chromosome.
– If two genes are close together, the recombination
frequency between them should be low, since
crossovers are rare.
– If they are far apart, recombination rates between
them should be high.
Gene Map of Drosophila
Warm Up Exercise
• If two genes are on the same chromosome but
usually assort independently, what does that
tell you about how close together they are?
Human Chromosomes
• Karyotypes- a picture of
chromosomes grouped together
in pairs.
– Humans have 46 chromosomes.
Two of them are sex
chromosomes, because they
determine an individual’s sex.
The remaining 44 chromosomes
are called autosomes.
– Females are XX and males are XY.
Human Traits
• Pedigree Charts- shows the relationships
within a family.
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Human Traits
• The phenotype of an organisms are only partly
governed by the genotype. Many traits are
strongly influenced by the environment,
nutrition, and exercise.
– Environmental effects are not inherited though,
only genes are inherited.
Human Genes
• The Human Genome- our complete set of
genetic information- includes thousands of
genes.
Human Blood Groups
• The Rh blood group is determined by a single
gene with two alleles- positive and negative.
– The positive Rh+ allele is dominant.
• 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, and produces no antigen.
Some Autosomal Disorders in Humans
Type of Disorder
Disorder
Disorders caused by Albinism
Recessive alleles
Disorders caused by
Dominant alleles
Major Symptoms
Lack of pigment in skin, hair, and eyes.
Cystic Fibrosis
Excess mucus in lungs, digestive tract, liver;
increased susceptibility infections.
Phenylketonuria
Accumulation of phenylalanine in tissues;
lack of normal skin pigment; mental
retardation
Achondroplasia
dwarfism
Huntington’s
Mental deterioration and uncontrollable
movements; symptoms usually appears in
middle age.
Disorders caused by Sickle Cell Disease Mishapen, or sickled, red blood cells’
Codominant alleles
damage to many tissues
From Gene to Molecule
• In some diseases, such as cystic fibrosis, and
sickle cell disease, a small change in the DNA
of a single gene affects the structure of a
protein, causing a serious genetic disorder.
Exit Slip
• If a woman with type O blood and a man with
type AB blood have children, what are the
children’s possible genotypes?
Warm Up
Need new warm up question
Human Genes and
Chromosomes
• Sex-Linked Genes- genes
located on the sex
chromosomes are said to be
sex-linked.
– Males have just one X
chromosomes, thus all X-linked
alleles are expressed in males,
even if they are recessive.
Inheritance of Colorblindness
Human Genes and
Chromosomes
• Barr Bodies- an inactive
form of an X chromosome in
females.
• Nondisjunction- when
homologous chromosomes
fail to separate in meiosis.
– Nondisjunction can lead to an
abnormal number of
chromosomes.
Exit Slip
• Distinguish between sex-linked disorders and
sex chromosome disorders.