Download Notes: Menedelian genetics

Vocabulary
C12L03C12
 Allele - alternative form that a single gene may
have for a particular trait (dominant &
recessive)
 Dihybrid – a genetic cross to examine the
possible inheritance of two specific alleles
(traits) (e.g. flower color & plant height)
 Dominant - The stronger of two genes expressed
in the hybrid; represented by a capital letter (R).
Masks recessive alleles.
 Codominance – Pattern where phenotypes of
both homozygous parents are produced in
heterozygous offspring so that both alleles for a
gene are equally expressed. (e.g. a cross between
a red cow (RR) and a white cow (WW) produce
red a white (RW) spotted cows)
 F1 generation - the first-generation
offspring in a breeding experiment.
(1st filial generation, your parents)
 F2 generation - the second-generation of
offspring in a breeding experiment.
(2nd filial generation, you)
 Gene - a unit of heredity; a sequence of
DNA nucleotides that determines a
single characteristic.
 Genetics - science of heredity
 Genotype – gene or allele combination
for a trait. This is the letter combination
for the genes. (e.g. RR, Rr, rr)
 Heredity - passing of traits from parent
to offspring
 Heterozygous or Hybrid - gene
combination of one dominant & one
recessive allele (e.g. Rr)
 Homozygous or true breeding (pure) gene combination involving 2 dominant
or 2 recessive genes (e.g. RR or rr)
 Incomplete Dominance – Pattern where one
allele is not completely dominant over the
other. Generally displays a new trait. (Parents =
Red Flower (RR), and White Flower (rr);
Offspring are all Rr with Pink flowers)
 Law of Independent Assortment - Mendelian
law stating that a random distribution of alleles
occurs during the formation of gametes.
 Law of Segregation - Mendelian law stating that
two alleles for each trait separate during
meiosis
 Monohybrid – a genetic cross involving a single
trait e.g. flower color
 Parental P1 Generation - the parental
generation in a breeding experiment.(your
grandparents)
 Pedigree – Graphic representation of
genetic inheritance used by geneticists to
map genetic traits.
 Phenotype – Physical expression of the
gene. The observable characteristic that is
expressed as a result of genotype (e.g. red,
white)
 Polygenic Traits – traits controlled by two
or more genes; genes may be on the same
or different chromosomes
 Punnett Square - a diagram used to analyze
the possible gene combinations of the
offspring between two individuals.
( genotypes and phenotypes)
 Recessive - gene that shows up less often in
a cross; represented by a lowercase letter
(r). Is not seen if a dominant allele is
present.
 Sex Linked Traits – traits controlled by
genes located on sex chromosomes
( X and Y chromosomes)
 Trait — characteristic that is inherited; can
be either dominant or recessive
Mendelian Genetics
 Known as the “father of genetics”
 Austrian Monk
 Studied the inheritance of traits in pea
plants
 Developed the rule of dominance and laws
of inheritance
 Mendel's work was not recognized until the
turn of the 20th century
 Can be grown in a
small area
 Produce lots of
offspring very quickly
 Produce pure plants
when allowed to selfpollinate several
generations
 Can be artificially
cross – pollinated
Phenotype
Genotype
1.
2.
3.
4.
5.
6.
7.
8.
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or Wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position ---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
He studied only one trait at a time to control
variables, and he analyzed his data
mathematically.
 Pollen contains
sperm which is
stamen
produced by the
stamen
 Ovary (found inside
the flower) contains
eggs
• Pollen carries sperm to the eggs for
fertilization
• Self-fertilization can occur in the same
flower
• Cross-fertilization can occur between
flowers
pistol
ovary
 Mendel cross-
pollinated flowers using
a paintbrush
 He could snip the
stamens to prevent
self-pollination
 He traced traits
through the several
generations
Mendel
produced pure
strains by
allowing the
plants to selfpollinate for
several
generations
Mendel’s first
experiments are
called monohybrid
crosses because
mono means “one”
and the two parent
plants differed from
each other by a
single trait like
height
Used to help solve genetic problems
 When recording the
results of crosses, it
is customary to use
the same letter for
different alleles of
the same gene.
 An uppercase letter is
used for the
dominant allele and a
lowercase letter for
the recessive allele.
 The dominant allele
is always written first.
Tall plant
Short plant
t t
T T
T
t
T
t
Mendel cross-pollinated a tall pea plant with
pollen from a short pea plant
T
Short
pea plant
t
t
T
Tall
pea plant
All of their offspring were as tall or taller than the
parent plants.
Short
pea plant
T
T
t
Tt
Tt
t
Tt
Tt
Tall
pea plant
These plants were allowed to self pollinate
T
Tall
pea plant
T
t
t
Tall
pea plant
Three-fourths of the plants were as tall as the
tall plants in the parent and first generations
and One-fourth of the offspring were as short as
the short plants in the parent generation.
Tall
pea plant
T
t
T
TT
Tt
t
Tt
tt
Tall
pea plant
• Parent generation (P1)
produced all tall pea
plants. (F1)
• Short phenotype
disappeared
• First generation (F1)
• A cross of two
offspring from P1
• Produce F2
• Second generation (F2)
• Short phenotype
reappears
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
F2
3 tall: 1 short
 In every case, he found that one trait of a
pair seemed to disappear in the F1
generation, only to reappear unchanged in
one-fourth of the F2 plants.
 Mendel concluded that each organism has
two factors that control each of its traits.
 We now know that these factors are genes
and that they are located on chromosomes.
 An organism’s two alleles are located on
different copies of a chromosome—one
inherited from the female parent and
one from the male parent.
 Mendel called the observed trait
dominant and the trait that disappeared
recessive.
Mendel’s experiments helped him
formulate the 2 laws of inheritance.
1. The law of segregation
And
2. The law of independent
assortment
States that every individual has two alleles of each
gene and when gametes are produced, each
gamete receives one of these alleles.
 During the formation of gametes (eggs or
sperm), the two alleles responsible for a trait
separate from each other.
 Alleles for a trait are then "recombined" at
fertilization, producing the genotype for
the traits of the offspring.
TALL
SHORT
Tt
Male
Parent
Tt
Female
Parent
T
t
T
TT
Tt
t
Tt
tt
Male
Gametes
Female
Gametes
DAD
MOM
YOU
Genotype: Uu
Phenotype:
two eyebrows
 States that genes for different traits—for example,
seed shape and seed color—are inherited
independently of each other.
 This law can be illustrated using dihybrid
crosses.
Monohybrid Crosses
Genotype Possibilities: RR, Rr, rr
Phenotype Possibilities: Red or White
Red (R) Dominant – White (r) Recessive
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Round seeds x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Genotype: Rr
Genotypic
Ratio: 4:4
Phenotype: Round
Rr
Phenotypic
Ratio: 4:4
33
 Homozygous dominant X
Homozygous recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is
ALL ALIKE 4:4
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Round seeds x Round seeds
Rr
x
Rr
Genotype: RR, Rr, rr
R
r
R
RR
Rr
Phenotype: Round &
wrinkled
r
Rr
rr
P.Ratio: 3:1
G.Ratio: 1:2:1
35
 Heterozygous X Heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1