Download Genetics

Mendelian Genetics
G.Burgess
2011.
Genetics

Genetics = the science of heredity that
involves the structure and function of genes
and the way genes are passed from one
generation to the next.

Hereditary trait = a characteristic under control
of the genes that is transmitted from one
generation to another.
Gregor Mendel
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Mendel was the first person to gain some
understanding of the principles of heredity.

1866 – he did research involving pea plants to
discover the mechanisms of genetics and
applied what he learned about heredity to
people and animals.
Used statistical analysis to help prove his
hypothesis

Homologous Pairs
Mendel understood that we get our
traites from both our mother and father.
Further research indicated that the
traites are located on chromosomes at
specific locations.
Our chromosomes are in pairs. Each
chromosome in the pair has genes in
the same location.
This is a homologous pair.
Why did Mendel use pea plants?

Easy to grow

Grow quickly
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Easy to crossbreed
Mendel’s Experiment
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Mendel obtained 34 strains of pea plants that
differed in a number of traits ( ie. seed shape,
seed color, height, flower color etc. )

Allowed strains to self-fertilize for many
generations to ensure pure-breeding strains
(ensures that the offspring will have the same
traits (genes) as the parents)
Mendel’s Experiment continued

Mendel specific traits (or physical
characteristics: phenotype) for study
– Flower color (white & purple)
– Seed color ( yellow& green)
– Seed shape ( smooth & wrinkled)
– Pod color ( green & yellow)
– Stem height ( tall & short)
Terminology

Before discussing Mendel’s experiments, you must
understand some basic terminology
– P generation = parental generation
– F1 generation = first set of offspring
– F2 generation = produced by breeding the F1
offspring
– Allele = a form of a gene (dominant or
recessive)
– Dominant = the allele that is expressed over
another
– Recessive = the allele that is ‘masked’
– X means crossed with…
Mendel’s Experiment

Crossed 2 true-breeding strains of peas that
differed in a single trait (monohybrid cross)
– i.e P =smooth seed plant X wrinkled seed
plant
– F1 = all smooth seeds
Mendel’s Experiment

Mendel’s second step = he allowed the F1
generation to self-fertilize to produce the F
2
– F1 X F1 ( smooth seeds)
– = F2 (3 smooth: 1 wrinkled)
– How can a trait be present in the P
generation disappear in the F1 and reappear
in the F2 ?
Dominant vs. Recessive Traits

In the case of the pea plants, there are 2
alternative forms (or alleles) for each
characteristic

FOR EXAMPLE the gene for determining the
shape of the seed has two forms (alleles);
• wrinkled and smooth.
Dominant & Recessive cont.

Recall the example;
– P = smooth X wrinkled = F1 (All smooth)
– *the allele for smooth is dominant because it
is “masking” the wrinkled gene
– *the allele for wrinkled is recessive because
in it is no shown
Dominant and Recessive

Dominant
– The expressed allele
(the one you can
see)
– Represented by a
capital letter
– ie S = smooth

Recessive
– The allele that is not
expressed ( or seen)
– Represented by
lower case letter
– ie s = wrinkled
Example of a Smooth vs Wrinkled Cross



Remember
S = smooth
s = wrinkled
(these are 2 alleles for the gene that
determines shape of the seed)
Each plant has 2 alleles for each gene (one
from the mother, one from the father:
homologous pair)
Example of a Smooth vs Wrinkled Cross
(continued)

Therefore, plants can be SS or Ss or ss

Homozygous = 2 copies of the same
allele (ie SS or ss) both are recessive or
dominant

Heterozygous = 2 different alleles (ie
Ss) 1 dominant and 1 recessive (the dominant trait is
shown in the phenotype
What would these plants look like?



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Wrinkled or smooth?
(**remember S = smooth, s = wrinkled)
SS = ?????
ss = ?????
Ss = ?????
Smooth
Wrinkled
Smooth
It’s a good time to talk about genotype and
phenotype...


Phenotype = the observable characteristics of
an organism (is smooth or wrinkled
appearance)
Genotype = the genetic make-up of an
organism
• (ie SS or Ss or ss)
IMPORTANT

*Note: A plant with different GENOTYPE (SS
and Ss) will have the SAME PHENOTYPE
(they will both appear wrinkled because the
smooth allele is dominant over the wrinkled)
Punnett Square
http://www.blc.arizona.edu/courses/1
81gh/rick/genetics1/punnet.html
PUNNETT SQUARE

Punnett square = a matrix that describes all the
possible gametic fusion that will give rise to the
zygotes that will produce the next generation
Punnett Square
F1 Gametes
Yy X Yy
♂Male
♀
Female
Y
Y
Y
YY
Yy
y
Yy
yy
Mendel’s Laws


Law of Segregation - during gamete
formation the pair of genes responsible for
each trait separates so that each gamete
receives only one gene for each trait
Law of Independent Assortment - genes for
different traits segregate independently during
gamete formation.
Two-Trait Crosses

Mendel also studied the inheritance of two
traits at once

(ie round yellow plants (RRYY) X wrinkled
green (wwyy)
Two-trait continued

Remember, the law of segregation stated that
each gamete must contain ONE allele from
each trait

therefore, the gametes of a round, yellow
parent are R, Y and the gametes of a wrinkled,
green parent are r, y.
Two-trait Cross example
http://mil.citrus.cc.ca.us/cat2courses/bio104/ChapterNotes/Chapter10notesLewis.htm
Incomplete Dominance



In all the examples we have seen this far, one
trait has been completely dominant over the
other (ie tall (T) is dominant over short(t))
Dominant and recessive traits are not always
clearly defined, incomplete dominance also
exists
Occurs when one allele only partially masks
another.
Incomplete Dominance





For example - four o’clock plants
they have red and white flowers (neither are
dominant)
red = RR
white = rr
When crosses, the Rr plants are pink
Incomplete Dominance
Multiple Alleles




Multiple Alleles = a set of three or more
different alleles controlling a trait.
ie human blood type is determined by three
alleles A, B & i
A and B are dominant over i
BUT A and B are codominant
Multiple Alleles (blood types)
Genotype
Phenotype
IAIA
IAi
A
IBIB
IBi
B
IAIB
AB
ii
O
•A is incompletely dominant over B.
•Blood typing is
done by examining
the surface
proteins found on
cells. Type IA blood
has antigen A.
Blood type IB has
antigen B. Type I
blood does not
have either
antigen.
•An individual with blood type AB has characteristics of
both blood types.
•A person who has blood type O does not have either
antigen.
•Blood type AB is also an example of codiminance
Codominance

Occurs when two alleles for a
genetic trait are neither dominant
nor recessive