Genetics
Patterns of Inheritance
Genetics
• In the 21st century is a generic term.
• Now have applied or statistical geneticscame from observation and Mendel
• Cytogenetics – study of chromosomes in
the cell & gene locations on chromosomes
• Molecular genetics- sequencing of nucleic
acids, genes sequences
Genetic Terminology
• Trait - any characteristic that can be
passed from parent to offspring
• Heredity - passing of traits from parent
to offspring
• Genetics - study of heredity
• Alleles - two forms of a gene (dominant
& recessive)
• Dominant - stronger of two genes
expressed in the hybrid; represented
by a capital letter (R)
• Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
• Genotype - gene combination for a trait
(e.g. RR, Rr, rr)
• Phenotype - the physical feature
resulting from a genotype (e.g. tall,
short)
• Homozygous genotype - gene combination
involving 2 dominant or 2 recessive genes
(e.g. RR or Rr); also called pure
• Heterozygous genotype - gene combination
of one dominant & one recessive allele (e.g.
Rr); also called hybrid
• Monohybrid cross - cross involving a single
trait
• Dihybrid cross - cross involving two traits
• Punnett Square - used to solve genetics
problems
For another view of the terms
• Go to Mendel’s Genetic Laws at
http://www.hobart.k12.in.us/jkousen/Biolog
y/mendel.htm
Blending Concept of Inheritance
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Accepted before Mendel's experiments
Theory stated that offspring would have traits
intermediate between those of its parents such as
red & white flowers producing pink
The appearance of red or white flowers again was
consider instability in genetic material
Blending theory was of no help to Charles Darwin's
theory of evolution
Blending theory did not account for variation and
could not explain species diversity
Particulate theory of Inheritance, proposed by
Mendel, accounted for variation in a population
generation after generation
Mendel's work was unrecognized until 1900
Mendelian Genetics
• 1862
1868
1880
Gregor Mendel
• Austrian monk – Abbot
• Studied science & math at the
University of Vienna
• Formulated the laws of heredity in the
early 1860's
• Did a statistical study of traits in
garden peas over an eight year period
Why peas, Pisum sativum?
• Can be grown in a
small area
• Produce lots of
offspring
• Produce pure plants
when allowed to
self-pollinate
several generations
• Can be artificially
cross-pollinate
Mendel's Experiments
• Mendel studied simple traits from 22 varieties of pea
plants (seed color & shape, pod color & shape, etc.)
• Mendel traced the inheritance of individual traits &
kept careful records of numbers of offspring
• He used his math principles of probability to
interpret results
• Mendel studied pea traits, each of which had a
dominant & a recessive form (alleles)
• The dominant (shows up most often) gene or allele is
represented with a capital letter, & the recessive
gene with a lower case of that same letter (e.g. B, b)
Mendel's traits included
a. Seed shape --- Round (R) or Wrinkled (r)
b. Seed Color ---- Yellow (Y) or Green (y)
c. Pod Shape --- Smooth (S) or wrinkled (s)
d. Pod Color --- Green (G) or Yellow (g)
e. Seed Coat Color --- Gray (G) or White (g)
f. Flower position --- Axial (A) or Terminal (a)
g. Plant Height --- Tall (T) or Short (t)
h. Flower color --- Purple (P) or white (p)
• Mendel produced pure strains by
allowing the plants to self-pollinate
for several generations
• These strains were called the
Parental generation or P1 strain
• Mendel cross-pollinated two
strains and tracked each trait
through two
generations (e.g. TT x tt )
Results of Mendel's Experiments
• Inheritable factors or genes are responsible for all
heritable characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one from the
mother and the other from the father
• True-breeding individuals are homozygous ( both
alleles) are the same
• Law of Dominance states that when different alleles
for a characteristic are inherited (heterozygous), the
trait of only one (the dominant one) will be
expressed. The recessive trait's phenotype only
appears in true-breeding (homozygous) individuals
Law of Dominance
Trait: Pod Color
Genotypes:
Phenotype:
Green Pod
GG
Green Pod
Gg
Yellow Pod
gg
Law of Segregation
• states that each genetic trait is produced
by a pair of alleles which separate
(segregate) during reproduction
Rr
R r
Law of Independent Assortment
• states that each factor (gene) is distributed
(assorted) randomly and independently of
one another in the formation of gametes
RrYy
RY Ry rY ry
This is what Mendel said
1) Dominant alleles overpower recessive
alleles. Dominant traits overpower
recessive traits.
Let's say that "B" means that a cat will be BIG.
Let's say that "bb" will make a small cat.
"BB" and "Bb" cats would show the dominant phenotype. There is no difference
between "Bb" and "bB".
"bb" cats show the recessive phenotype.
• 2) Rule of segregation: Gametes (sex
cells) only receive one allele from the
original gene.
• 3) Rule of Independent assortment:
One trait will not determine the random
selection of another.
Independent assortment
Summary of Laws
LAW
Parent Cross Offspring
DOMINANCE
TT x tt
tall x short
100% Tt
tall
SEGREGATION
Tt x Tt
tall x tall
75% tall
25% short
INDEPENDENT RrGg x RrGg
ASSORTMENT round & green x
round & green
9/16 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
Beyond Mendelian Genetics
• Not all patterns of inheritance obey the
principles of Mendelian genetics. In fact, many
traits observed are due to a combined
expression of alleles.
• 1. Incomplete dominance or blending
inheritance: the traits will blend, cross a white
and a red snapdragon get pink offspring.
• 2. Co dominance:.
• 3. Linked Genes
• In 1940, the famous geneticist Alfred Sturtevant noted that about
70% of people of European ancestry are able to roll up the lateral
edges of the tongue, while the remaining 30% were unable to do so.
Tongue rolling ability may be due to a single gene with the ability to
roll the tongue a dominant trait and the lack of tongue rolling ability a
recessive trait. However, there is some question about the
inheritance of tongue rolling. Recent studies have shown that
around 30% of identical twins do not share the trait.
•
• If earlobes hang free, they are detached. If they attach directly to the
side of the head, they are attached earlobes.
Some scientists have reported that this trait is due to a single gene
for which unattached earlobes is dominant and attached earlobes is
recessive. Other scientists have reported that this trait is probably
due to several genes.
The size and appearance of the lobes are also inherited traits.
What are some Mendelian Traits
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Have freckles (F) vs No freckles (f)
Dimples (D) vs No dimples (d)
Widow’s peak (W) vs Straight (w)
Have chin cleft (C) vs No chin cleft (c)
Hitchhiker’s Thumb (H) vs Straight (h)
In clasped hands is Left thumb on top (L)
Right thumb on top (l)
Other Patterns of Inheritance
• Incomplete dominance occurs in the heterozygous or hybrid
genotype where the 2 alleles blend to give a different phenotype
• Flower color in snapdragons shows incomplete dominance
whenever a red flower is crossed with a white flower to produce
pink flowers
Co-dominance
an equal expression of
both alleles.
Example, an individual
can have an AB blood
type. Each allele is
equally expressed
Alleles A & B are
dominant, while O is
recessive
Genotype Phenotype
OO
II
Type O
AO
II
Type A
AA
II
Type A
IBIO Type B
BB
II
Type B
AB
I I Type AB
Linked genes
• Each chromosomes has 1000's of genes
• All genes on a chromosome form a linkage group
that stays together except during crossing-over
• Some genes located on the same chromosome tend
to be inherited together
• Linked genes were discovered by Thomas Hunt
Morgan while studying fruit flies
• Linked alleles do not obey Mendel's laws because
they tend to go into the gametes together
• Crosses involving linked genes do not give same
results as unlinked genes
Linked genes
• sometimes genes on the same
chromosome stay together during
assortment and move as a group. The
group of genes is considered linked and
tends to be inherited together. Example,
genes for flower color and pollen shape
are linked on the same chromosomes and
show up together.
Sex-linked traits
• Autosomes and sex chromosomes are the
two types of chromosomes found in
eukaryotes. The autosomes code for most
traits and protein in the body. The other
pair – the sex chromosomes determine
the sex of an individual. Some traits, such
as color blindness and hemophilia are
carried on the sex chromosomes. These
are called sex linked traits.
Now Think about genes
• What if the genetic information in
each family member were like a
jigsaw puzzle? Each puzzle piece
would represent a set of genes
organized in a specific way, similar to
a chromosome. Because all humans
have the same set of genes,
arranged in the same order, every
family member would have the same
basic set of puzzle pieces. A generic
human jigsaw puzzle might look like
the picture at the right
• But the information carried in genes
differs slightly from person to
person. This is what makes each of
us unique. As a result, the colors of
the puzzle pieces would be different
between family members. While
some relatives might share puzzle
pieces of a certain color, other
pieces would be different. Only
identical twins share the exact same
combination of colors and shapes.
• What might a family's puzzles
look like?
• Look at the family of jigsaw puzzles
right. Can you see how some of the
child's genes are derived from one
parent and some from the other
parent?
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The child receives exactly half of its
genetic information from the mother and
exactly half from the father
http://learn.genetics.utah.edu/archive/pedigree /
Chromosomes & Human
Inheritance
• Chromosomes:
• Thomas Sutton in 1902 proposed that genes are
located on chromosomes
• Called the Chromosome Theory of Inheritance
• For most of the life of the cell, chromosomes are too
elongated to be seen under a microscope &
are called chromatin
• Before a cell gets ready to divide, each chromosome
is duplicated & condenses into short structures
• Each chromosome is composed of a single, tightly
coiled DNA molecule
• The two DNA
strands are
homologous
(duplicates) and are
held together by the
centromere
• While they are still
attached, the
duplicated
chromosomes are
called sister
chromatids
review
• Fertilization restores the diploid
chromosome number and paired
condition for alleles in the zygote
• Chromosomes can be categorized as
two types --- autosomes & sex
chromosomes
• Autosomes are non-sex chromosomes
that are the same number and kind
between sexes
• Sex chromosomes determine if the
individual is male or female
• Sex chromosomes in the human female
are XX and those of the male are XY
• Males produce X-containing and Ycontaining gametes; therefore males
determine the sex of offspring
Pedigrees
• Also called a family tree
• Squares represent males and circles
represent females
• Horizontal lines connecting a male and
female represent mating
• Vertical lines extending downward from
a couple represent their children
• A shaded symbol means the individual
possess the trait
Half-shaded symbols are carriers
•
An example of genotype and the corresponding phenotype
chart for ABO blood type, which is a co-dominant trait
.
Sex Linkage
• Thomas Hunt Morgan worked with fruit flies &
confirmed that genes were on chromosomes
a. Fruit flies are cheaply raised in common
laboratory glassware
b. Females only mate once and lay hundreds of eggs
c. Fruit fly generation time is short, allowing rapid
experiments
• Experiments involved fruit flies with XY system
similar to human system
• Besides genes that determine sex, sex
chromosomes carry many genes for traits unrelated
to sex
• Thomas Hunt Morgan worked
with fruit flies & confirmed
that genes were on
chromosomes
a. Fruit flies are cheaply raised
in common laboratory
glassware
b. Females only mate once and
lay hundreds of eggs
c. Fruit fly generation time is
short, allowing rapid
experiments
• Experiments involved fruit flies
with XY system similar to
human system
• Besides genes that determine
sex, sex chromosomes carry
many genes for traits unrelated
to sex
• Cross of white-eyed male with dominant red-eyed
female yield expected 3:1 red-to-white ratio;
however, all white-eyed flies were males
• An allele for eye color on the X but not Y
chromosome supports the results of the cross
• Heterozygous females are carriers that do not show
the trait but can pass it on
• Males are never carriers but express the one allele
on the X chromosome
• Red-green color-blindness is X-linked recessive
• In humans, another well-known X-linked
traits is hemophilia (free bleeders that lack
clotting factors in their blood)
• One of the most famous genetic cases
involving hemophilia goes back to Queen
Victoria who was a carrier for the disorder
and married Prince Albert who was normal
• Their children married other royalty, and
spread the gene throughout the royal
families of Europe
Example Sex-Linked Problems
1. What are the results of crossing a
colorblind male with a female carrier
for colorblindness?
• Trait: Red-Green Color blindness
Alleles: XC normal vision
Xc colorblindness
XCXc
x
Xc Y
Red green color blindness
C
X
C
X
C C
Y Genotypes:
Genotypic
XX XY
Ratio:
C C
C
Xc XCXc XcY Phenotypes:
C
C c
X X ,X Y, X X ,
XcY
1:1:1:1
normal vision female,
normal vision male,
female carrier,
colorblind male
Chromosome Mutations
• Mutations are changes in genes or chromosomes
that can be passed on to offspring
• Mutations increase the number of variations that
occur
• Chromosomal mutations include changes in
chromosome number and/or structure
• Monosomy occurs when an individual has only one
of a particular type of chromosome
• Turner syndrome (X0) is an example of monosomy
• Trisomy occurs when and individual has three of a
particular type of chromosome
• Examples of trisomy include Klinefelter's Syndrome
(XXY) and Down Syndrome or Trisomy 21 where the
individual has three 21st chromosomes
• Both monosomy & trisomy result when
chromosomes fail to separate during meiosis; called
nondisjunction
• Monosomy and trisomy (aneuploidy) occur in plants
and animals and may be lethal (deadly)
• Polyploidy where the offspring have more than two
sets of chromosomes occurs often in plants (3n, 4n
...)
• Environmental factors including radiation,
chemicals, and viruses, can cause chromosomes to
break causing a change in chromosomal structure
Activity time
• Class activity – practicing using a Punnett
square
• We will use two worksheets front in class
and the back is homework to be turned in
next Monday.
Mendelian or Punnett Squares
• Setting up and using a Punnett square is
quite simple once you understand how it
works. You begin by drawing a grid of
perpendicular lines:
• Genetic
Tic tac toe
• Next, you put the genotype of one parent
across the top and that of the other parent
down the left side. For example, if parent
pea plant genotypes were YY and GG
respectively, the setup would be:
• Note that only one letter goes in each box
for the parents. It does not matter which
parent is on the side or the top of the
Punnett square.
• Next, all you have to do is fill in the boxes
by copying the row and column-head
letters across or down into the empty
squares. This gives us the predicted
frequency of all of the potential genotypes
among the offspring each time
reproduction occurs
Completed square
• if the parent plants both have heterozygous (YG)
genotypes, there will be 25% YY, 50% YG, and 25% GG
offspring on average. These percentages are
determined based on the fact that each of the 4 offspring
boxes in a Punnett square is 25% (1 out of 4). As to
phenotypes, 75% will be Y and only 25% will be
G. These will be the odds every time a new offspring is
conceived by parents with YG genotypes.
• An offspring's genotype is the result of the combination
of genes in the sex cells or gametes (sperm and ova)
that came together in its conception. One sex cell came
from each parent. Sex cells normally only have one
copy of the gene for each trait (e.g., one copy of the Y or
G form of the gene in the example above). Each of the
two Punnett square boxes in which the parent genes for
a trait are placed (across the top or on the left side)
actually represents one of the two possible genotypes for
a parent sex cell. Which of the two parental copies of a
gene is inherited depends on which sex cell is inherited-it is a matter
Why is it important for you to know about Punnett squares? The
answer is that they can be used as predictive tools when
considering having children. Let us assume, for instance, that
both you and your mate are carriers for a particularly
unpleasant genetically inherited disease such as cystic
fibrosis . Of course, you are worried about whether your
children will be healthy and normal. For this example, let us
define "A" as being the dominant normal allele and "a" as the
recessive abnormal one that is responsible for cystic
fibrosis. As carriers, you and your mate are both
heterozygous (Aa). This disease only afflicts those who are
homozygous recessive (aa). The Punnett square below
makes it clear that at each birth, there will be a 25% chance of
you having a normal homozygous (AA) child, a 50% chance
of a healthy heterozygous (Aa) carrier child like you and your
mate, and a 25% chance of a homozygous recessive (aa)
child who probably will eventually die from this condition.
If both parents are carriers of the recessive
allele for a disorder, all of their children will
face the following odds of inheriting it:
25% chance of having the recessive disorder
50% chance of being a healthy carrier
25% chance of being healthy and not have
the recessive allele at all
• If a carrier (Aa) for such a recessive
disease mates with someone who has it
(aa), the likelihood of their children also
inheriting the condition is far greater (as
shown below). On average, half of the
children will be heterozygous (Aa) and,
therefore, carriers. The remaining half will
inherit 2 recessive alleles (aa) and develop
the disease.
If one parent is a carrier and the other has a
recessive disorder, their children will have the
following odds of inheriting it:
50% chance of being a healthy carrier
50% chance having the recessive disorder
• It is likely that every one of us is a carrier for
a large number of recessive alleles. Some of
these alleles can cause life-threatening
defects if they are inherited from both
parents. In addition to cystic fibrosis,
albinism, and beta-thalassemia are recessive
disorders.
• Some disorders are caused by dominant
alleles for genes. Inheriting just one copy of
such a dominant allele will cause the
disorder. This is the case with Huntington
disease, achondroplastic dwarfism, and
polydactyly. People who are heterozygous
(Aa) are not healthy carriers. They have the
disorder just like homozygous dominant (AA)
individuals.
Heredity and Traits
• http://learn.genetics.utah.edu/content/begi
n/traits/
• Genetic tutorials
• http://www.serpwidgets.com/Genetics/gen
etics.html
• Some scientists have reported that
handedness is due to a single gene with
right handedness dominant and left
handedness recessive. However, other
scientists have reported that the
interaction of two genes is responsible for
this trait.
• If only one parent has a single copy of a
dominant allele for a dominant disorder,
their children will have a 50% chance of
inheriting the disorder and 50% chance
of being entirely normal.
• Interactive practice
http://www.athro.com/evo/gen/punexam.ht
ml
• Shockwave tutorial
How many sex chromosomes are in a human gamete
http://www2.edc.org/weblabs/Punnett/pun
nettsquares.html
• Reginald Punnett