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Patterns of Inheritance
Mendelian Inheritance
Key concepts
• Mendel’s research showed that traits are
inherited as discrete units.
• Mendel’s data revealed patterns of inheritance
• The same gene can have many versions
• Genes influence the development of traits
• The inheritance of traits follow the rules of
probability
• A monohybrid cross involves one trait
• A dihybrid cross involves two traits.
Case study: Sudden Death on the
Court
• http://www.clevelandleader.com/node/6446
• Michael Phelps'
• Read Sudden death on the court of Flo Hyman
Handout to be made from the ninth Edition
• Share your understanding of genetic disorder
with your partner
What is the Physical Basis of
Inheritance?
Genes are sequences of Nucleotides at specific
locations on chromosomes.
• A chromosome consists of a single double helix of
DNA made up of nucleotides.
• Genes the units of inheritance are parts of
chromosomes.
• A gene’s physical location on a chromosome is
called locus.
• The chromosomes of diploid organisms occur in
pairs called homologues.
Alleles
• Both members of a pair of homologues carry
the same genes, located at the same loci.
However, the nucleotide sequences of a given
gene may differ in different members of a
species, or even on the two homologues of a
single individual. These different versions of a
gene at a given locus are called alleles.
• The relationships among genes, alleles, and
chromosomes can be seen in the next slide.
Both chromosomes carry the
same allele of the gene at this
locus, and so the organism is
homozygous at this locus
Each chromosomes carries a
different allele of this gene, so
the organism is heterozygous at
this locus
Mutations are the source of Alleles
• Alleles arise as mutations that slightly change the
nucleotide sequences.
• If a mutations occurs in the cells that become
sperm or eggs, it can be passed on from parent to
offspring.
• Most of the alleles in an organism’s DNA first
appeared as mutations in the reproductive cells
of the organism’s ancestors, perhaps millions of
years ago, and have been inherited generations
after generations.
Homozygous and Heterozygous
• A diploid organism has pairs of homologous
chromosomes, and both members of a pair contain the
same gene loci and so the organism has two copies of
each gene.
• If both homologues have the same allele at a given
gene locus, the organism is said to be homozygous at
that locus
• If two homologous chromosomes have different alleles
at a locus, the organism is heterozygous at that locus.
• The organism that are heterozygous at a specific locus
are often called hybrids
Homozygous and Heterozygous
The Secrets of Mendel’s Success
The three key steps to Mendel’s Success
• Choosing the right organism with which to work
• Designing and performing the experiment correctly
• Analyzing the data correctly
Parts of edible pea flower
Useful link to know about Mendel’s Experiment
Edible Pea flower
• Mendel chose the edible pea as the subject for his
experiments.
• In pea flowers, the petals enclose all of the
reproductive structures, preventing another flower’s
pollen from entering.
• Therefore the egg cells in a pea flower must be
fertilized by sperm from the pollen of the same flower
and this process is called self-fertilization.
• Mendel also opened the pea flower and removed its
stamens, preventing self fertilization. Then he dusted
the sticky tip of the carpel with pollen from the flower
of another plant and this is cross fertilization
Traits of Pea Plants studied by Mendel
Mendel chose to study individual characteristics (called traits) that had different forms
such as white versus purple flowers. He started out to study one trait at a time. He
followed the inheritance of these traits for several generations, counting the number of
offspring with each type of trait.
By analyzing these numbers, the basic patterns of inheritance became clear
4.3.1 Define genotype, phenotype, dominant allele, recessive allele,
recessive allele, codominant alleles, locus, homozygous, heterozygous,
carrier, and test cross
• Genotype:
• Genotype: The symbolic representation of a pair of
alleles possessed by an organism, typically
represented by two letters.
• The genetic constitution of an organism
• Examples: TT, Tt, tt.
Phenotype
• Phenotype: The characteristics or trait of an
organism- the way in which the genotype is expressed
(appearance of an organism)
• Examples: five fingers on each hand, color blindness.
Definitions
• Dominant allele
• An allele that has the same effect on the phenotype whether it is
paired with the same allele or a different one. Dominant alleles
are always expressed in the phenotype.
• An allele that has the same effect on the phenotype in a
heterozygous individual (where it is combined with a recessive
allele) as in a homozygous individual (where there are two copies
of the dominant allele)
• Recessive allele: an allele that has an effect on the phenotype in
homozygous individuals (where there are two copies of the
recessive allele). In heterozygous individuals the recessive allele is
hidden by the dominant allele.
• Homozygous: having two identical alleles of a
gene.
• Examples: TT is a genotype which is homozygous
dominant whereas tt is the genotype of
someone who is homozygous recessive for that
trait.
• Heterozygous: Having two different alleles of a
gene.
• Example: Tt is a heterozygous genotype.
• Test cross: testing a suspected heterozygote by
crossing it with a known homozygous recessive.
Definitions
• Codominant alleles: pairs of alleles that both
affect the phenotype when present in a
heterozygote.
• Locus: the particular position on homologous
chromosome of a gene. Each gene is found at
a specific place on a specific pair of
chromosome. So alleles of the same gene
occupy the same locus.
Monohybrid Cross Animation
• http://www.siskiyous.edu/class/bio1/genetics
/monohybrid_v2.html
MonoHybrid Crosses: a cross involving one trait
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Experiment: Crossing a homozygous purple flowered plant
with a homozygous white flowered plant
Appearance = phenotype
Genetic make-up = genotype
F1 = a hybrid formed by crossing
differing pure-breeding parents.
Homozygous = both alleles the same.
Hetrozygous = both alleles the different.
F2 = the offspring produced by
inbreeding the F1.
4.3.2 Determine the genotypes and phenotypes of the
offspring of a monohybrid cross using a Punnet grid.
Monohybrid Cross
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We can study one of the characteristics Gregor Mendel used in his experiments.
He studied the size of pea plants and found that ‘tall’ is dominant over ‘short’. If
we start the experiment with 2 pure breeding (homozygous) plants of contrasting
traits (tall and short), we will obtain an F1 (First Filial generation) which has the
dominant phenotype (tall) but is heterozygous.
When self-fertilizing the F1, we will obtain an F2 (Second filial generation) which
will appear ¾ dominant (tall) and ¼ recessive (short)
Start by writing the phenotypes for the relevant gene and what the
corresponding genotypes are as in the following table.
Possible phenotypes
Corresponding genotypes
Tall
TT or Tt
Short
Tt
Then write out the cross. Make sure to include both genotypes and phenptypes
of the parents (P), as well as the genotype and phenptype of the offsproing (F1).
Monohybrid Cross
http://www.jdenuno.com/PDFfiles/Monohybrid.pdf
You can go to this site and have practice for Monohybrid cross using Punnet grid.
Drag and drop Genetics Monohybrid
• http://www.zerobio.com/drag_gr11/mono.ht
m
• Practice Monohybrid cross by using this link
Incomplete Dominance
Incomplete Dominance
• When the heterozygous phenotypes is intermediate between the
two homozygous phenotypes, the pattern of inheritance is called
incomplete dominance.
• In humans, hair texture is influenced by a gene with two
incompletely dominant alleles which we call C1 and C2.
• A person with two copies of the Copies of the C1 allele has curly
hair; two copies of C2 allele produce straight hair.
• Heterozygotes, with C1C2 genotype have wavy hair.
• Work out the cross between two parents having wavy hair and
show parental, F1, and F2 offspring.
Laws of Mendel
Law of Segregation
The pairs of alleles on homologous chromosomes
separate, or segregate, from each other during
meiosis. As a result, each gamete receives only
one allele of each pair
Mendel's law of independent assortment, states
that allele pairs separate independently during
the formation of gametes. This means that traits
are transmitted to offspring independently of one
another.
• http://www.siskiyous.edu/class/bio1/genetics
/dihybrid_v2.html
• Animation on Dihybrid cross
Dihybrid cross genetic diagram
F1 Phenotypes
RrYy
Genotypes
Round Yellow
Proportions
100%
(Selfed)
meiosis
Gametes
Y
y
R
RY
Ry
r
rY
ry
Dihybrid cross genetic diagram
Gametes
Y
y
R
RY
Ry
r
rY
ry
fertilisation
F2
Genotypes
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Dihybrid cross proportions
Phenotypes
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Proportions
9/16 or 56.25%
Dihybrid cross proportions
Phenotypes
Round Yellow
Proportions
9/16 or 56.25%
Round Green
3/16 or 18.75%
Wrinkled Yellow
3/16 or 18.75%
Wrinkled Green
1/16 or 6.25%
Dihybrid test cross
• In monohybrid crosses, to know if a
dominant trait is homozygous (RR) or
heterozygous (Rr) it is necessary to carry out
a test cross
• This is done with a homozygous recessive
(rr) individual
• The same is true for a dihybrid cross where
the test cross is made with an individual
which is homozygous recessive for both
characters (rryy)
Dihybrid test cross
Phenotypes
Genotypes
Round Yellow
RrYy
Gametes
RY, Ry, rY, ry
Genotypes
ry
Phenotypes
X
Wrinkled Green
rryy
ry
RY
Ry
rY
ry
RrYy
Rryy
rrYy
rryy
Round
Round
Wrinkled
Wrinkled
Yellow
Green
Yellow
Green
Proportions 1/4 or 25% 1/4 or 25% 1/4 or 25% 1/4 or 25%
Dihybrid Cross Problem
• In summer squash, white fruit color (W) is dominant
over yellow fruit color (w) and disk-shaped fruit (D) is
dominant over sphere-shaped fruit (d).. If a squash
plant true-breeding for white, disk-shaped fruit is
crossed with a plant true-breeding for yellow, sphereshaped fruit, what will the phenotypic and genotypic
ratios be for:
• a. the F1 generation? b. the F2 generation?
http://www.personal.psu.edu/hw7/Biology110/dihybrid.
htm
You can check your answer
Useful links and reference
• http://www.siskiyous.edu/class/bio1/genetics/di
hybrid_v2.html
• Animation on Dihybrid cross
• http://www.zerobio.com/drag_gr11/mono.htm
• Drag and drop Genetics Monohybrid
• http://www.dnalc.org/resources/animations/
• Animations on Genetics
•