Honors Biology
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Background:
 Deduced the fundamental principles of genetics by
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breeding garden peas.
Known as the “Father of Genetics”
Was a monk that lived and worked in an abbey in
Austria.
In a paper in 1866, Mendel correctly argued that
parents pass on to their offspring discrete heritable
factors.
In his paper, he stressed that the heritable factors
(today called genes) retain their individuality
generation after generation.
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Experiments:
 Chose to study garden peas because he was familiar
with them from his rural upbringing, they were easy
to grow, and they came in many readily
distinguishable varieties.
 Also, he was able to exercise strict control over pea
plant matings.
▪ That is, sperm-carrying pollen grains released from the
stamens land on the egg-containing carpel of the same
flower.
▪ He used a small bag to cover the flower to ensure selffertilization.
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Experiments (continued)
 Due to their anatomical nature (petals of pea
flower almost completely enclose the stamen and
carpel), pea plants usually self-fertilize in nature.
 He was also able to ensure cross-fertilization
▪ Fertilization of one plant by pollen from a different
plant.
 Through his methods, Mendel could always be
sure of the parentage of new plants
 He chose seven characteristics, that occur in two
distinct forms ,to study:
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Flower color (purple, white)
Flower position (axial, terminal)
Seed color (yellow, green)
Seed shape (round, wrinkled)
Pod shape (inflated, constricted)
Pod color (green, yellow)
Stem length (tall, dwarf)
 Mendel worked with his plants until he was sure
he had true-breeding varieties.
▪ For instance, he identified a purple-flowered variety
that, when self-fertilized, produced offspring that all
had purple flowers.
 He then asked, What offspring would result if plants with purple
flowers and plants with white flowers were cross-fertilized?
▪ The offspring of two different varieties are called hybrids, and the
cross-fertilization itself is referred to as a hybridization, or simply a
cross.
▪ The true-breeding parental plants are called the P generation ( P
for parental).
▪ The offspring of the P generation are called the F1 generation (F for
filial, the Latin word “son”)
▪ When the F1 self-fertilize or fertilize with each other, their offspring
are called the F2 generation.
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Mendel performed many experiments in
which he tracked the inheritance of
characteristics that occur in two forms, such
as flower color.
Monohybrid cross:
 When you’re looking only at one trait (ex, flower
color)
 Mendel performed a monohybrid cross between a pea plant with purple
flowers and one with white flowers.
▪ The F1 offspring all had purple flowers (not a lighter purple, has predicted
by a “blending” hypothesis.)
▪ Was the white gene lost?
▪ By mating the F1 plants, Mendel found the answer to be NO!
▪ Out of 929 F2 plants, Mendel found that 705 (about ¾) had purple
flowers and 224 (about ¼) had white flowers, a ratio of about three
plants with purple flowers to one with white flowers in the F2 generation
(3:1)
▪ The heritable factor for white flowers did not disappear in the F1 plants,
but the purple-flower factor was the only one affecting the F1 flower
color.
▪ The F1 plants must have carried two factors for the flower-color
characteristic, one for purple and one for white.
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Mendel observed these same patterns of
inheritance for six other pea plant
characteristics.
From these results, he developed four
hypotheses, which we will describe using
modern terminology (such as “gene” instead
of “heritable factor”):
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Hypothesis 1:
 There are alternative forms of genes that account
for variations in inherited characteristics.
 For example, the gene for flower color in pea
plants exists in two forms, one for purple and the
other for white.
 The alternative versions of a gene are now called
alleles.
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Hypothesis 2:
 For each characteristic, an organism inherits two
alleles, one from each parent. These alleles may be
the same ore different.
 An organism that has two identical alleles for a
gene is said to be homozygous for that gene (and
is called a homozygote).
 An organism that has two different alleles for a
gene is said to be heterozygous for that gene
(and is called a heterozygote)
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Hypothesis 3:
 If the two alleles of an inherited pair differ, then one
determines the organism’s appearance is called the
dominant allele; the other has no noticeable effect
on the organism’s appearance and is called the
recessive allele.
 We use upper-case letters to represent dominant
alleles and lowercase letters to represent
recessive alleles.
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Hypothesis 4:
 A sperm or egg carries only one allele for each
inherited trait because allele pairs separate
(segregate) from each other during the production
of gametes.
 This statement is now known as the law of
segregation.
 When sperm and egg unite at fertilization, each
contributes its allele, restoring the paired
condition in the offspring.
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Also…
Law of independent assortment – during
gamete formation (meiosis), alleles of
DIFFERENT traits are arranged
independently form one another
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The right hand side of the diagram on the
previous slide explains the results of Mendel’s
experiment.
 In this example, P represents the dominant allele (for
purple flowers) and p represents the recessive allele
(for white flowers).
 At the top, you see the alleles carried by the parental
plants both were true-breeding.
 Mendel proposed that one parent had two alleles for
purple flowers (PP) and the other had two alleles for
white flowers (pp)
 Mendel’s hypotheses also explain the 3:1 ratio in
the F2 generation; because the F1 hybrids are Pp,
they make gametes P and p in equal numbers.
 You can see the possible gamete combinations
using a Punnett square.
▪ Used to make predictions about the possible
phenotypes and genotypes of offspring.
 Consistent with hypothesis 4, the gametes of
Mendel’s parental plants each carried one allele;
thus, the parental gametes in Figure 9.3B are
either P or p.
 As a result of fertilization, the F1 hybrids each
inherited one allele for purple flowers and one for
white.
 Hypothesis 3 explains why all of the F1 hybrids are
(Pp) had purple flowers; the dominant P allele has
its full effect in the heterozygote, while the
recessive p allele had no effect on flower color.
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Genotype vs. Phenotype
 Phenotype
▪ For example, purple or white flowers.
▪ Term used to describe an organism’s appearance, or
expressed physical traits.
▪ Phenotypic ratio- ratio of the phenotypes of the offspring.
▪ Example, the ratio of purple flowers to white flowers is 3:1
 Genotype
▪ For example, PP, Pp, or pp.
▪ Term used to describe an organism’s genetic makeup.
▪ Genotypic ratio- ratio of the genotypes of the offspring.
▪ Example, the 1:2:1 is the ratio of PP, Pp, pp
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Remember, two homologous chromosomes may bear either
the same alleles or different ones. Thus, we see the
connection between Mendel’s laws and homologous
chromosomes: Alleles (alternate forms) of a gene reside at the
same locus on homologous chromosomes.
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Test Cross:
 a mating between an individual of unknown
genotype and a homozygous recessive individual.
 Mendel used testcrosses to determine whether he
had true-breeding varieties of plants.
 Continues to be an important tool of geneticists
for determining genotypes.
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Dihybrid cross:
 Results from a mating of parental varieties differing in two
characteristics.
 For example: Mendel crossed homozygous round yellow seeds (RRYY)
with plants having wrinkled green seeds (rryy).
▪ All of the offspring in the F1 generation had round yellow seeds;
which raised the question: are the two characteristics transmitted
from parent to offspring as a package, or was each characteristic
inherited independently of the other?
▪ The question was answered when Mendel allowed fertilization to
occur among the F1 plants the offspring supported the idea that
the two seed characteristics segregated independently.
▪ Offspring had nine different genotypes, and four different
phenotypes with 9:3:3:1 ratio.
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Mendel’s results supported the hypothesis
that each pair of alleles segregates
independently of the other pairs of alleles
during gamete formation Mendel’s Law of
Independent Assortment
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Although Mendel’s laws are valid for all sexually
reproducing organisms, they stop short of
explaining some patterns of genetic
inheritance.
In fact, for most sexually reproducing
organisms, cases where Mendel’s laws can
strictly account for the pattern of inheritance
are relatively rare.
More often, the inheritance patterns are more
complex…
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Complete dominance
 The dominant allele had the same phenotype
whether present in one or two copies.
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Incomplete dominance
 The F1 hybrids have an appearance in between the
phenotypes of the two parental varieties.
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For example, red snapdragons crossed with
white snapdragons produced hybrid flowers
in the F1 with pink flowers.
 This third phenotype results from flowers of the
heterozygote having less pigment than the red
homozygotes.
 The F2 generation would then have a 1:2:1 ratio of
red, pink, white.
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In humans, an example involves the condition
hypercholestrolemia, dangerously high levels of cholesterol
in the blood, caused by a recessive allele(h), due to a lack of
LDL receptors.
hh individuals have about 5 times the normal amount of
blood cholesterol and may have heart attacks as early as age
2.
Normal individuals are HH.
Heterozygotes have blood cholesterol about twice normal.
 Usually prone to atherosclerosis, the blockage of arteries
by cholesterol buildup in artery walls, and they may have
heart attacks from blocked heart arteries by their mid-30s.
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Both alleles are expressed in the
heterozygous individual
Different from incomplete dominance, which
is the expression of one intermediate trait
Can be seen in blood type
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Both alleles are expressed in the
heterozygous individual
Different from incomplete dominance, which
is the expression of one intermediate trait
Can be seen in blood type
The ABO blood group phenotype in humans
involves three alleles of a single gene.
 These three alleles, in various combinations,
produce four phenotypes: a person’s blood group
may be either O, A, B, or AB.
 These letters refer to two carbohydrates,
designated A and B, that may be found on the
surface of red blood cells.
 A person’s red blood cells may have carbohydrate A
(type A blood), carbohydrate B (type B), both (type
AB), or neither (type O).
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Matching compatible blood groups is critical
for safe blood transfusions.
If a donor’s blood cells have carbohydrate (A
or B) that is foreign to the recipient, then the
recipient’s immune system produces blood
proteins called antibodies that bind
specifically to the foreign carbohydrates and
cause donor blood cells to clump together,
potentially killing the recipient.
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Four blood groups result from various combinations of the three
different alleles, symbolized as IA, IB, and i.
Each person inherits one of these alleles from each parent.
IA and IB are dominant to the i allele, but are codominant to each other =
both alleles are expressed in the heterozygote IAIB , who have the blood
type AB
There are six possible genotypes:
 IAIA and IAi= A
 IBIB and IBi= B
 IAIB= AB
 ii = O
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Most genes influence multiple characteristics,
a property called pleiotropy.
An example of pleiotropy in humans is sicklecell disease
 Refer to p. 168
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Polygenic inheritance is the additive effects
of two or more genes on a single phenotypic
characteristic
 Examples include human skin color and height
 Different then pleiotropy, in which a single gene
affects several characteristics
The frequency of traits with polygenic
inheritance follow the shape of a bell
curve.
Besides bearing genes that determine sex, the sex
chromosomes also contain genes for characteristics
unrelated to femaleness and maleness.
 Sex-linked genes are genes located on either sex
chromosomes, although in humans the term has
historically referred specifically to a gene on the X
chromosome. ***Be careful not to confuse the term
sex-linked gene with the term linked genes!!!***
 Refer to figure 9.23A-D on p. 176
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Many characteristics result from a combination of heredity and
environment.
For example, in humans nutrition influences height, exercise alters build,
sun-tanning darkens the skin, and experience improves performance on
intelligence tests.
It is becoming clear that human phenotypes—such as risk of heart
disease and cancer and susceptibility to alcoholism and schizophrenia—
are influenced by both genes and environment.
Simply spending time with identical twins will convince anyone that
environment, and not just genes, affect a person’s traits.
However, only genetic influences are inherited…cannot pass on
environmental influences to future generations!
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The chromosome theory states that genes
occupy specific loci (positions) on
chromosomes and it is the chromosomes that
undergo segregation and independent
assortment during meiosis.
Thus, it is the behavior of chromosomes
during meiosis and fertilization that accounts
for inheritance patterns.
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Because of the chromosomal theory, genes
located close together on the same
chromosome tend to be inherited together
and are called linked genes.
Linked genes generally do not follow
Mendel’s law of independent assortment.
Refer to figure 9.19 in book
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As we saw in meiosis, crossing over between
homologous chromosomes produces new
combinations of alleles in gametes
Forms recombinant gametes
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Pedigree is a family tree used to study how
particular human traits are inherited.
It is analyzed using logic and the Mendelian
laws
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1. Determine the mode of inheritance:
dominant, or recessive, sex-linked or
autosomal
2. Determine the probability of an affected
offspring for a given cross.
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Is it a dominant pedigree or a recessive pedigree?
1. If two affected people have an unaffected child, it must be a dominant
pedigree: D is the dominant allele and d is the recessive allele. Both
parents are Dd and the normal child is dd.
2. If two unaffected people have an affected child, it is a recessive
pedigree: R is the dominant allele and r is the recessive allele. Both
parents are Rr and the affected child is rr.
3. If every affected person has an affected parent it is a dominant
pedigree.
I
2
1
II
1
2
3
4
5
6
III
1
2
3
4
5
6
7
8
9
10
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1. All unaffected are dd.
2. Affected children of an affected parent and an unaffected parent must
be heterozygous Dd, because they inherited a d allele from the
unaffected parent.
3. The affected parents of an unaffected child must be heterozygotes Dd,
since they both passed a d allele to their child. (also called carriers)
4. If both parents are heterozygous Dd x Dd, their affected offspring have
a 2/3 chance of being Dd and a 1/3 chance of being DD.
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1. all affected are rr.
2. If an affected person (rr) mates with an unaffected person, any
unaffected offspring must be Rr heterozygotes, because they got a r
allele from their affected parent.
3. If two unaffected mate and have an affected child, both parents must
be Rr heterozygotes.
4. Recessive outsider rule: outsiders are those whose parents are
unknown. In a recessive autosomal pedigree, unaffected outsiders are
assumed to be RR, homozygous normal.
5. Children of RR x Rr have a 1/2 chance of being RR and a 1/2 chance of
being Rr. Note that any siblings who have an rr child must be Rr.
6. Unaffected children of Rr x Rr have a 2/3 chance of being Rr and a 1/3
chance of being RR.
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In any pedigree there are people whose parents are unknown. These
people are called “outsiders”, and we need to make some assumptions
about their genotypes.
Sometimes the assumptions are proved wrong when the outsiders have
children. Also, a given problem might specify the genotype of an
outsider.
Outsider rule for dominant pedigrees: affected outsiders are assumed to
be heterozygotes. (or carriers)
Outsider rule for recessive pedigrees: unaffected (normal) outsiders are
assumed to be homozygotes.
Both of these rules are derived from the observation that mutant alleles
are rare.
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All unaffected individuals are homozygous for the normal
recessive allele.
Example: Huntington’s Disease
Look for:
 Trait in every generation
 Once leaves the pedigree does not return
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Every person with the trait must have a parent with the
trait
Males and females equally affected
Look for:
 Skips in generation
 Unaffected parents can have affected children
 Affected person must be homozygous
 Males and females affected equally
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The recessive gene is located on 1 of the autosomes
Letters used are lower case ie bb
Unaffected parents (heterozygous) can produce affected offspring (if they
get both recessive genes ie homozygous)
Inherited by both males and females
Can skip generations
If both parents have the trait then all offspring will also have the trait. The
parents are both homozygous.
E.g. cystic fibrosis, sickle cell anaemia, albanism
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Genes are carried on the sex chromosomes
(X or Y)
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Sex-linked notation
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XBXB normal female
XBXb carrier female
XbXb affected female
XBY normal male
XbY affected male
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Mothers pass their X’s to both sons and daughters
 If the mother has an X- linked dominant trait and is homozygous
(XAXA) all children will be affected
 If Mother heterozygous (XAXa) 50% chance of each child being
affected
Fathers pass their X to daughters only.
 Affected males pass to all daughters and none of their sons
▪ Genotype= XAY
Normal outsider rule for dominant pedigrees for females, but for sexlinked traits remember that males are hemizygous and express whichever
gene is on their X.
XD = dominant mutant allele
Xd = recessive normal allele
E.g. dwarfism, rickets, brown teeth enamel.
Look for:
 More males being affected
 Affected males passing onto all daughter
(dominant) and none of his sons
 Every affected person must have an
affected parent
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Look for:
More males being affected
Affected female will pass onto all her sons
Affected male will pass to daughters who
will be a carrier (unless mother also
affected)
Unaffected father and carrier mother can
produce affected sons
Examples: Hemophilia, Colorblindness