Download Mendelian Genetics and crosses

Mendelian Genetics
An Overview
• Pea plants have several advantages for genetics.
– Pea plants are available in many varieties with distinct
heritable features (characters) with different variants (traits).
– Another advantage of peas is that Mendel had strict control
over which plants mated with which.
– Each pea plant has male
(stamens) and female
(carpal) sexual organs.
– In nature, pea plants typically
self-fertilize, fertilizing ova
with their own sperm.
– However, Mendel could also
move pollen from one plant
to another to cross-pollinate
plants.
Fig. 14.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In a typical breeding experiment, Mendel would
cross-pollinate (hybridize) two contrasting,
true-breeding pea varieties.
– The true-breeding parents are the P generation and
their hybrid offspring are the F1 generation.
• Mendel would then allow the F1 hybrids to selfpollinate to produce an F2 generation.
• It was mainly Mendel’s quantitative analysis of
F2 plants that revealed the two fundamental
principles of heredity: the law of segregation
and the law of independent assortment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. By the law of segregation, the
two alleles for a characters are
packaged into separate gametes
• If the blending model were correct, the F1 hybrids
from a cross between purple-flowered and whiteflowered pea plants would have pale purple
flowers.
• Instead, the F1 hybrids
all have purple flowers,
just a purple as the
purple-flowered
Fig. 14.2
parents.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• When Mendel allowed the F1 plants to selffertilize, the F2 generation included both purpleflowered and white-flowered plants.
– The white trait, absent in the F1, reappeared in the
F 2.
• Based on a large
sample size, Mendel
recorded 705
purple-flowered F2
plants and 224
white-flowered F2
plants from the
original cross.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 14.2
Vocabulary
• Character –heritable feature
• Trait – each variant for a character
• True-breeding – plants that self-pollinate all
offspring are the same variety
• Monohybrid cross – a cross that tracks the
inheritance of a single character
• P generation – (parental) true-breeding
• F1- (first filial) offspring of P generation
• F2 – (second filial) offspring from F1 cross
Vocabulary (continued)
•
•
•
•
Allele- alternate version of a gene
Dominate allele – expressed in the heterozygote
Recessive allele – not expressed in the heterozygote
Homozygote – pair of identical alleles for a
character
– Homozygous dominant- BB
– Homozygous recessive - bb
• Heterozygote – two different alleles for a character
(Bb)
• Genotype – genetic makeup
• Phenotype – appearance of an organism
Law of Segregation - the two alleles for each
character segregate during gamete
production
Law of Independent Assortment – Each
set of alleles segregates independently
Test cross – designed to reveal the
genotype of an organism
Mendelian Inheritance and Rules
of Probability
• Rule of Multiplication – the probability
that two events will occur simultaneously is
the product of their individual probabilities
• Probability that an egg from the F1 (Pp)
will receive p = ½
• Probability that an sperm from the F1 (Pp)
will receive p = ½
• Probability that a of offspring receiving
two recessive alleles during fertilization
½x½=¼
Dihybrid crosses
Two factor crosses
Rule Applies to dihybrid Crosses
• For a dihybrid cross, YyRr x YyRr, what is
the probability of an F2 having the genotype
YYRR? Dihybrid Crosses: Crosses that
involve 2 traits.
• .
• These type of crosses can be challenging to
set up, and the square you create will be
4x4. This simple guide will walk you
through the steps of solving a typical
dihybrid cross common in genetics. The
method can also work for any cross that
involves two traits
• Consider this cross
• A pea plant that is heterozygous for round,
yellow seeds is self fertilized, what are the
phenotypic ratios of the resulting offspring?
• Step 1: Determine the parental genotypes
from the text above, the word "heteroyzous"
is the most important clue, and you would
also need to understand that self fertilized
means you just cross it with itself.
• Step 2: Determine the gametes. This might
feel a little like the FOIL method you
learned in math class. Combine the R's and
Ys of each parent to represent sperm and
egg. Do this for both parents
• Gametes after "FOIL"
• RY, Ry, rY, ry (parent 1) and RY, Ry, rY,
ry (parent 2)
• Step 3: Set up a large 4x4 Punnet square,
• Step 4: Write the genotypes of the offspring
in each box and determine how many of
each phenotype you have. In this case, you
will have 9 round, yellow; 3 round, green; 3
wrinkled, yellow; and 1 wrinkled green
• Some Shortcuts
• In any case where the parents are
heterozygous for both traits (AaBb x AaBb)
you will always get a 9:3:3:1 ratio.
Incomplete Dominance
Page 267 and work #2
Codominance
• Two alleles affect the phenotype in separate
and distinguishable ways.
• Neither allele can mask the other and both
are expressed in the offspring and not in an
“intermediate” form.
• Example: red flowers that are crossed with
white flowers that yield red and white
flowers.
• 1) In cattle, roan coat color (mixed red and white
hairs) occurs in the heterozygous (Rr) offspring of
red (RR) and white (rr) homozygotes. When two
roan cattle are crossed, the phenotypes of the
progeny are found to be in the ratio of 1 red:2
roan:1 white. Which of the following crosses
could produce the highest percentage of roan
cattle?
• A) roan x roan
• B) red x white
• C) white x roan
• D) red x roan
• E) All of the above crosses would give the same
percentage of roan.
Multiple Alleles
Page 267 and work #6
Pleiotropy
• Most genes have
multiple
phenotypic effects.
The ability of a
gene to affect an
organism in many
ways is called
pleiotropy.
Epistasis
• Epistasis occurs
when a gene at one
locus alters or
influences the
expression of a gene
at a second loci. In
this example, C is for
color and the
dominate allele must
be present for
pigment (color) to be
expressed.
Polygenetic Inheritance
• Qualitative variation
usually indicates
polygenic inheritance.
This occurs when there
is an additive effect
from two or more
genes. Pigmentation in
humans is controlled by
at least three (3)
separately inherited
genes.
Environmental Impact on Phenotype
pH of the soil will change the color of
hydrangea flowers from blue to pink
Technology
And
Genetic testing
Fetal testing
Carrier
Recognition
1.
Amniocentesis
2.
Chorionic villus
sampling (CVS)
3.
Ultrasound
4.
Fetoscopy
Newborn
screening