Transcript Fundamentals of Genetics

Fundamentals of
Genetics
Ch. 11
Gregor Mendel
– Monk who taught high school science and mathematics
– He kept a garden plot at the monastery where he did his
science work
– Father of genetics for his work on pea plants and
heredity.
– Heredity: the transmission of characteristics from
parents to offspring.
Mendel’s Garden Peas
– He observes 7 characteristics of pea plants
– Each characteristic occurred in two contrasting traits
– Trait: a category in which alternate characteristics can be observed.
–
Plant height – long vs. short stems
–
Flower position – axial vs. terminal
–
Pod color – green vs. yellow
–
Pod appearance – inflated vs. constricted
–
Seed texture – smooth vs. wrinkled
–
Seed color – yellow vs. green
–
Flower color – purple vs. white
Mendel’s First Observations
– He took seeds and noted from which plants they were obtained
– Ex: He took purple flower plant seeds and crossed them with some
of the same type
– He noticed that these plants had purple flowers, but they would
also have offspring with white flowers.
– He noticed this for each one of the traits.
– He wanted to know why this happened
Mendel’s Methods
– Pollination – occurs when pollen grains
produced in the male reproductive parts of
a flower, called the anthers, are transferred
to the female reproductive part of a flower,
called the stigma.
– He carefully controlled pollination by
removing the anthers after he self
pollinated the plants he wanted to cross.
– Self pollination occurs when pollen is
transferred from the anthers of a flower to
the stigma of the same flower or of one on
the same plant. (Pea plants usually
reproduce this way)
– Cross pollination involves flowers of two
separate plants.
Mendel’s Experiment
–
Mendel first grew plants that were pure for a trait
–
Pure: always produce offspring with the desired trait.
–
Strain: denotes plants that are pure for a specific trait.
–
He produced strains by only allowing the plants to self-pollinate.
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He eventually obtained 14 strains, one for each of the traits he observed.
–
Each strain was called the parental generation, or P1 generation.
–
He then cross-pollinated these strains with plants that were pure for the opposite trait to produce the F1
generation.
–
He collected the seeds from the F1 generation and allowed them to mature and recorded his observations.
–
He then self pollinated the flowers from the F1 generation to produce the offspring to the F2 generation.
–
He allowed these plants to mature and recorded once again his observations.
–
He did this several hundreds of times with each of the traits recording his observations each time.
Mendel’s Results and Conclusions
– Each time he crossed the P1 generations, the F1 generation always
resulted in only one of the two traits being seen physically.
– Each time he crossed the F1 generations, the F2 generation always
had the ratio of 3:1
– He hypothesized that the trait appearing in the F1 generation was
controlled by a DOMINANT factor, because it masked, or dominated,
the other characteristic.
– The trait that did not appear he thought was controlled by a RECESSIVE
factor.
Chromosomes and Genes
– Law of Segregation – a pair of factors is segregated, or separated, during the formation
of gametes.
– Law of Independent Assortment – factors for different characteristics are distributed to
gametes independently. (one factor is inherited separately from another)
– Because chromosomes occur in pairs, genes also occur in pairs. Each of several forms of
a gene is called an allele.
– Mendel’s factors are now called alleles.
– The dominant allele for a genes is given a capital letter, whereas the recessive form of
an allele is given a lowercase letter.
– Ex: Tall is dominant over short, so we use the letter t. T = tall, t = short
Genetic Crosses
– The genetic makeup of an organism is its genotype. (The allele letter
combinations)
– The appearance of an organism is called its phenotype.
– When both alleles of a pair are alike, the organism is said to be homozygous
for that characteristic.
– An organism can be homozygous dominant, or homozygous recessive.
– Ex: TT or tt
– Heterozygous is when the two alleles in the pair are different. Heterozygotes
are also referred to as carriers, as they carry the recessive gene, but do not
express it.
– Ex: Tt
Probability
– Probability – the likelihood that a specific event will occur.
– Expressed as a decimal, a percentage, or a fraction
– Probability = # of x’s an event is expected to happen
# of opportunities for an event to happen
– In Mendel’s experiments the number of yellow seed color appeared in the F2
generation 6,022 times and the total number of seeds was 8,023
– 6,022/8,023 = .75 or 75% or 3/4
– The green seeds appeared 2,001 times
– 2,001/8023 = .25 or 25% or ¼
Predicting Results of Monohybrid Crosses
– A cross between individuals that involves one pair of
contrasting traits is called a monohybrid cross.
– Punnett squares are used to show the possible
outcomes, or probability that a certain trait will be
inherited by the offspring.
– The following is an example of a cross between a
pure/homozygous purple (PP) flowering plant and a
pure/homozygous white flowering plant (pp).
– Outcome is 100% probability of getting purple
flowers
– Genotype is Pp and Phenotype is purple
Predicting Results of Monohybrid Crosses
– A Heterozygous purple flowering
plant is X with a Heterozygous
purple flowering plant. What will
the expected phenotype and
genotype ratios be?
– Genotype ratio is written as:
1 PP: 2 Pp: 1 pp
– Phenotype ratio is written as:
3 purple: 1 white
Your Turn
– A Homozygous guinea pig for black coat color is crossed
with a heterozygous for black coat color.
(Bb)
B
– We can see that black is the dominant trait since heterozygous
still results in black color.
–
Genotype ratio - 2 BB: 2 Bb
–
Phenotype ratio - 4:4 or 100% black
– Unknown dominant genotypes can be determined by crossing
it with a homozygous recessive…this is called a testcross.
BB
Bb
BB
Bb
B
– What letter shall we use? B (for black), so BB X Bb (bb is
brown)
– What will the genotype and phenotype ratios be?
b
(BB)
B
Incomplete Dominance
– So far we have talked about COMPLETE DOMINANCE alleles, one
form of the gene is dominant over the other.
– Sometimes F1 offspring will have a phenotype in between that of
the parents. This is known as INCOMPLETE DOMINANCE.
– Some flowers have alleles where both red and white both
influence the color but neither is dominant over the other. The
result when a homozygous red and a homozygous white is
crossed is all pink flowers.
– When two pink flowers are crossed the genotype ratio is: 1 RR:
2Rr: 1 rr
– So phenotype ratio is 1 red: 2 pink: 1 white
Codominance
– Codominance occurs when
both alleles for a gene are
expressed in a heterozygous
offspring.
– In codominance neither
allele is dominant or
recessive over the other.
– Nor do they blend in the
phenotype.
– They are both expressed.
Multiple Allele Traits and Polygenic
Traits
– Multiple allele traits are controlled by three or more alleles of the same gene that
code for a single trait.
– Ex: blood type
– Polygenetic Traits – A trait that is controlled by two or more genes.
– Ex: skin color, eye color, height (but height is also environmental controlled)
Human Blood Type
– There are 4 types of human blood – A, B, O, and AB.
– AB is a codominant form since both A and B are both dominant.
– O is the recessive form of blood.
– There is also something called the RH factor. Which means that you can also have a positive
or a negative blood type.
– Interesting Facts:
– Blood types A and B make antibodies against the other dominant type of blood. Neither
makes antibodies for O.
– AB does not make any antibodies against A nor B since it has both types.
– O makes antibodies against blood types A and B.
– O is the universal donor (O-more specifically), which means everyone can receive blood
from somebody with O blood type or from their own blood type.
– O can only receive blood from O.
How do Read Blood Type Results
Predicting Results of Dihybrid Crosses
– A dihybrid is a cross between
individuals that involves two
pairs of contrasting traits.
– In this example we start with
a homozygous dominant
yellow seed that is round and
it is crossed with a
homozygous recessive green
seed that is wrinkled
– All of the F1 generation with
be 100% round and yellow
and 100% RrYy
Heterozygous F1 Cross
– When the F1 heterozygous generation is
crossed, the results are as follows.
– Phenotype ratio is:
9 yellow/round: 3 green/round:
3 yellow/wrinkled: 1 green/wrinkled
– The genotype is more complicated ;-)
1 RRYY: 2 RRYy: 2 RrYY: 4 RrYy: 1 RRyy:
2 Rryy: 1 rrYY: 2 rrYy: 1 rryy
Sex Determination
– Like other chromosomes the sex
chromosomes form pairs.
– In mammals and most insects, males
have one X chromosome and one Y
chromosome.
– Females have two X chromosomes.
– Males determines the sex of the
offspring.
– Offspring have a 50% chance of either
being a boy or a girl.
Sex-Linked Traits
– More genes are carried on the X gene due to size difference!
– Genes carried on the X chromosome are said to be X-linked genes.
– Genes found on the Y chromosome are Y-linked.
– The presence of genes on the sex chromosomes is called sex linkage.
– Examples: Colorblindness, hemophilia, muscular dystrophy
– Sex-influenced traits are controlled by hormones, both males and females may have the same
genotype but different phenotype.
– Ex: male pattern baldness
– Males either have the disorder or do not due to size of the Y chromosome (it can not cover up
what is on the X chromosome if it does not have a second allele)
– Females can have three genotypes two which say they do not have the physical trait and one
that does. However, the heterozygous form is called a carrier, cause they can pass on the
gene to their sons.
Hemophilia Examples
– Ex: 1
– How many children have
hemophilia?
– What percentage of boys
have hemophilia?
– Ex: 2
– What percentage of
children have hemophilia?
– What percentage of girls
are carriers?