Mendelian
Inheritance
Biology In Focus Chapter 11
AP Biology 2014
Ms. Eggers
Learning Objectives
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Define Mendel's law of segregation.
Contrast dominant alleles with recessive alleles.
Define genotype and phenotype.
Explain how the law of probability applies to genetics.
Describe how a Punnett square can be used to predict the results of a genetic
cross.
Describe how a pedigree may be used to determine the mode of inheritance
of a genetic trait.
List some well-known genetic disorders in humans.
Explain the inheritance pattern of traits where more than two alleles for the
trait exist.
Define incomplete dominance and incomplete penetrance.
Define what is meant by pleiotropy.
Describe the inheritance of polygenic traits.
Explain X-linked inheritance and give some examples of X-linked traits in
humans.
Gregor Mendel (1822-1884)
 Austrian monk
 Biology & math geek? Yup!
 Explored heredity: the
passage of characters (a
heritable feature that varies
among individuals) from
parents to offspring.
 Looked at TONS of pea
plants for a variety of
different traits and came to
conclusions about the
transmission of characters:
Mendel’s Principles of
Heredity.
Mendel’s choice of pea plants as his
experimental model was key
 Pea plants reproduce quickly
 The mating of plants could be precisely controlled
 Peas demonstrate multiple traits that occur in two,
distinct forms
Crossing
pea plants
Dichotomous traits in peas
The Monohybrid Cross
 Monohybrid: the two
plants crossed were both
“true-breeding” but
contrasting for one single
trait.
 True-breeding: all of the
offspring display only one
form of the trait.
 Mating two true-breeding
varieties is called
hybridization.
 What are the names of
the various generations?
What did he conclude from the
results of his monohybrid crosses?
 First – in the F1 generation the white
flower disappeared. But it reappeared in
the F2 generation! This gave Mendel the
notion that hereditary factors remain
discreet – and do not blend. He called
the trait present in the F1 the dominant
character and the trait that was disguised
during the F1 the recessive trait.
Hereditary factor = gene
 Genes come in variations
 The variation of a gene is called an allele
 Genes/alleles for the same trait are located in the same
position on homologous chromosomes
What else did he conclude
from the monohybrid cross?
 Second – when he counted the number of
purple flowers and white flowers in the F2
generation, he saw 705 purple and 224
white flowered plants. He consistently saw
that the dominant and recessive traits
appeared in a 3:1 ratio. Another way of
saying this is that ¾ of the plants had
purple flowers and ¼ of the plants had
white flowers.
In total, he looked at over 21,000 hybrids
pea plants for these experiments!
Mendel knew his math…
 Mendel took this information and said
to himself, “Self. I know a little bit about
probability. If each parent has 2 copies
of the “hereditary factor” for that trait
but they only give one copy to their
offspring (so the offspring gets one from
each parent), then it’s just the flip of a
coin which copy the offspring gets from
each parent… So…
Genetics? Or calculating the odds in Vegas?
 If a parent has genotype Aa
(is heterozygous), she can
give either A or a to her
offspring and its just a 50:50
chance – or ½.
 Same for the other parent…
 The probability that the
offspring will get any
combination is the product
of the two separate
probabilities (½ x ½ = ¼).
Vocabulary
 Heredity: the passing of characters from parents to offspring
 Genetics: the study of heredity
 True-breeding: all of the offspring of a self-cross in plants
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display only one form of the character
P generation, F1 generation, F2 generation
Alleles: different versions of a gene
Dominant: the expressed form of a characteristic
Recessive: the form of the trait not expressed when the
dominant allele is present
Homozygous: having two of the same allele
Heterozygous: having two different alleles
Genotype: refers to the combination of alleles
Phenotype: the physical appearance of a characteristic
This handy picture is a Punnett Square –
and it shows what probability predicts.
Monohybrid cross ratios
 The ratio of phenotypes in the F2
generation is 3:1 (or ¾ are the
dominant phenotype and ¼ is the
recessive phenotype).
 The ratio of genotypes is 1:2:1 for
AA:Aa:aa
Remember meiosis?
Mendel’s law of segregation
 The two alleles for a
character segregate
(separate) when
gametes are formed.
 In other words, during
meiosis I the homologous
chromosomes separate so
that each gamete only gets
one copy of any
chromosome/gene.
Dihybrid cross: two
distinct traits at a time
Dihybrid cross ratios
 Ratio of phenotypes is 9:3:3:1 of
plants with both dominant traits:
one dominant and the other
recessive: one dominant and the
other recessive the other way: and
both recessive.
 Again, probability shows why!
Think of each trait separately
 For round vs. wrinkled seeds… in the F2 generation
¾ offspring will be round and ¼ will be wrinkled.
 For yellow vs. green seeds… in the F2 generation ¾
offspring will be round and ¼ will be wrinkled.
 So the chances of getting both round and yellow
(both dominant) is ¾ x ¾ = 9/16. The chances of
getting round and green is ¾ x ¼ = 3/16. The
chances of getting wrinkled and yellow is ¼ x ¾ =
3/16. And the chances of getting both green and
wrinkled (both recessive) is ¼ x ¼ = 1/16.
 This gives a ratio of 9:3:3:1.
Each trait is separate – Mendel’s law
of independent assortment.
What if you have a purple pea plant?
 Can you know if it’s AA
or Aa? Not by
looking…
 But, you can do a test
cross, which is crossing
the unknown genotype
with a pea plant that
shows the recessive
phenotype (and thus
known genotype of aa).
Dihybrid test cross
Lucky
 Mendel got lucky. He chose the
perfect experimental model with
which to work. Peas show
numerous traits with discreet
completely dominant and recessive
phenotypes.
 Inheritance patterns for most traits
are usually more complex
Incomplete dominance
 Snap dragons exhibit
incomplete dominance of
flower color
 Incomplete dominance:
when offspring have a
phenotype that lies
somewhere between the
two phenotypes of the
parents
 The Pp offspring make less
red pigment than the PP
plants & thus appear pink
Codominance
 The two alleles
affect the
phenotype in
separate,
distinguishable
ways
Homework
 Explain how the inheritance of the
human disease Tay-sachs
demonstrates distinct dominantrecessive phenotype, incomplete
dominance AND codominance at
various levels. (p. 216)
Multiple alleles – human blood types
Pleiotropy
 When a gene has
multiple
phenotypic effects
Even more complicated – when multiple
genes interact to confer a given phenotype
 Epistasis: the
phenotypic expression
of a gene at one locus
alters than of a gene at
a second locus
 Lab coat color – the
gene for pigment
deposition (E/e) is
epistatic to the gene
that codes for black or
brown pigment (B/b)
Polygenic inheritance
 Quantitative characters
vary in a population
along a continuum –
such as human height
or human skin color
 This usually indicates
polygenic inheritance =
multiple genes
contributing to a single
phenotype
Pedigree analysis
 Following human inheritance
through generations can be
informative
 And some human traits do show
simple Mendelian inheritance
Inherited human disorders
 Cystic fibrosis: lethal
recessive disorder. 1:25 people
of European descent are
carriers for this recessive
allele. Results in defective
chloride transport channels
 Achondroplasia: a dominant
trait
 Huntington’s disease: a
dominantly inherited disorder
that results in
neurodegeneration and death
Sickle cell anemia