Transcript Traits, Genes and Alleles

Genetics
Chromosomes and Meiosis
□ Two major groups of cells:
Somatic or Body Cells
Gametes or Sex Cells
□Make up most of your
body tissues and organs.
□DNA from these cells
is not passed on to your
children
□Considered diploid (2
copies of each
chromsome)
□Mitosis
□Includes eggs and
sperm
□DNA is passed on to
your children
□Considered haploid (1
copy of each
chromosome)
□Meiosis
Chromosomes and Meiosis
□ Each body cell contains a set of 46
chromosomes, which comes in 23 pairs
□ 23 from your mother
□ 23 from your father
□ They are genetically identical to each other
unless mutations occur
□ These are considered homologous
chromosomes
□ They have the general appearance and length
□ Copies of the same genes
Autosomes
•Contain genes for characteristics not related to
the sex of the organism (1-22)
Sex Chromosomes
•X and Y
cells.
□ Meiosis I occurs after DNA has been replicated.
□ Meiosis I divides homologous chromosomes in
four phases.
□ Meiosis II divides sister chromatids in four
phases.
□ DNA is not replicated between meiosis I
and meiosis II.
Meiosis and Genetic Variation
□ Genetic Variation
□Sexual reproduction creates unique
combination of genes.
□Independent assortment of
chromosomes in meiosis
□Random fertilization of gametes
Meiosis and Genetic Variation
□ Genetic Variation
□ Process called
crossing over also
creates genetic
variation
□ It is the exchange of
chromosome
segments between
homologous
chromosomes
□ Takes place during
meiosis
Crossing Over
Mendel and Heredity
□ Importance of his work:
□Disproved the thought that parents’
traits were blended in offspring
□Showed that organisms inherit two
copies of each gene from the parental
generation
□Law of segregation
Mendel and Heredity
□ Law of Segregation
□When an organism produces
gametes, each pair of alleles is
separated and each gamete has an
equal chance of receiving either one
of the alleles
Traits, Genes and Alleles
□ Genes
□ Pieces or segments
of DNA
□ Provides a set of
instructions to a cell
to make a certain
protein
□ Each has a specific
locus or position on
a a pair of
chromosomes
□ Can exist in many
forms
Traits, Genes and Alleles
□ Alleles
□ Alternative forms of a gene that may occur at a specific
locus
Traits, Genes and Alleles
□ A dominant allele is
expressed as a
phenotype when at
least one allele is
dominant.
□ A recessive allele is
expressed as a
phenotype only
when two copies
are present.
Traits, Genes and Alleles
□ Dominant alleles are represented by
uppercase letters while recessive alleles are
represented by lowercase letters
□ Dominant doesn’t mean the trait is better
or stronger
□ It doesn’t necessarily occurs most often in a
population
□ Dominant simply because it is expressed and
the other allele is not
Traits, Genes and Alleles
□ Genotype
□ Refers to the genetic
makeup of a set of
genes
□ Example:
□ BB tells you that this
organism is
homozygous for the
dominant trait
□ Phenotype
□ Refers to the physical
or observable traits of
an individual
□ Example:
□ Blue or brown eyes,
purple or white
flowers
Genotype: BB, Bb, bb
Phenotype: purple and white
Traits and Probability
Punnett Squares are used to illustrate
genetic crosses
The letters
represent the
alleles of the
parents
Traits and Probability
□ Monohybrid Crosses
□ Used to examine the inheritance of only one
specific trait
□ Example:
Dihybrid Crosses
Crosses that involve 2 different traits
Traits and Probability
□ Law of Independent Assortment
□States that different traits appear to be
inherited separately
□The inheritance of one trait does not
influence the inheritance of another trait
□Must a round seed also be yellow?
Chromosomes and Phenotype
Mendel’s rules of inheritance apply to autosomal
genetic disorders
– A heterozygote for a recessive disorder is a carrier.
– Disorders caused by dominant alleles are
uncommon.
(dominant)
Chromosomes and Phenotype
□ Males and females can
differ in sex-linked
traits.
□ Genes on sex
chromosomes are
called sex-linked
genes.
□ Y chromosome
genes in mammals
are responsible for
male
characteristics.
□ X chromosome
genes in mammals
affect many traits.
Males have
XY genotype.
Females have
XX genotype.
Complex Patterns of Inheritance
□Most traits aren’t simply dominant
or recessive
□Examples include:
□Incomplete dominance
□Codominance
□Multiple alleles
□Polygenic Traits
Complex Patterns of Inheritance
□ Incomplete
Dominance
□ The phenotype of the
heterozygotes is
intermediate between
2 homozygotes
□ Neither allele of the
pair is completely
dominant
□ Example: pink
flowers
Green betta
fish is
homozygous
for the green
color allele
The steel
blue is
homozygous
for the blue
color allele
The royal
blue fish is
heterozygous
for the two
color alleles
Complex Patterns of Inheritance
□ Codominance
□ Alleles are neither
dominant or recessive
□ Both alleles
contribute to the
phenotype of the
organism
□ Human blood types
is an example
Codominance
Gene I
is the gene that
determines blood
type
It has three alleles
Complex Patterns of Inheritance
Homozygous red flowers are crossed with
homozygous white flowers
□ Incomplete
Dominance
□ Offspring would
have pink
flowers
□ Codominance
□ Flowers would
have some red
areas and some
white areas
Complex Patterns of Inheritance
□ Polygenic Traits
□ Traits are
controlled by two
or more genes
□ Human skin color
and eye color
Order of dominance:
brown > green > blue.
Genes and the Environment
• Phenotype is a
combination of
genotype and
environment.
• The sex of sea turtles
depends on both
genes and the
environment
• Height is an example
of a phenotype
strongly affected by
the environment
Human Genetics
□ Pedigrees
□ A graphic representation of genetic
inheritance
□ It is a family history that shows how a trait
is inherited over several generations
□ Autosomal genes show different
patterns on a pedigree than sex-linked
genes.
□ If the phenotype is more common in
males, the gene is likely sex-linked.
Human Genetics
□ A karyotype is a picture of all
chromosomes in a cell.
XY
Mutations
□ A mutation is a change in an organism’s
DNA.
□ Many kinds of mutations can occur,
especially during replication.
□ Some mutations affect a single gene, while
others affect an entire chromosome.
□ Can be caused by errors in replication,
chemicals and UV radiation
Mutations
•Point Mutations
•Substitutes one nucleotide for another
mutated
base
Mutations
•Frameshift Mutations
•Inserts or deletes a nucleotide in the DNA
sequence.
Mistakes in Meiosis
□ Nondisjunction
□ Occurs when homologous chromosomes
do not separate correctly
□ Trisomy
□A gamete with an extra chromosome is
fertilized by a normal gamete and the
resulting zygote has and extra
chromosome
Mistakes in Meiosis
□ Trisomy 21
□ Commonly
called Down
Syndrome
□ Individuals
inherit 3 copies
of the 21st
chromsome
Mistakes in Meiosis
□ Nondisjunction:
□ Monosomy
□When a gamete with a missing
chromosome fuses with a normal gamete
□Most zygotes do not survive
□Example of non-lethal is Turner Syndrome
□Females have only a single X
chromosome