Name: ____________________________________________
Genetics – Ms. Mullin
Chapter 4 – Mendelian Inheritance – Notes
Inheritance
 Parents and offspring often share observable traits
 Grandparents and grandchildren may share traits not seen in parents
 Why do traits disappear in one generation and reappear in another?
Gregor Mendel (1822-1884)
 Father of Genetics
 Responsible for the laws governing inheritance of traits
 1822 – Born in what is now the Czeck Republic
 Grew up in an agricultural environment
 Joined St.Augustinian Monastery
 Higher studies at Vienna University
 1854 - Science teacher
 Carried out “hybridization” experiments with the common garden pea plants
 His hypotheses became the laws of inheritance in modern genetics
 Mendel cultivated and tested some 28,000 pea plants
 He found that the plants' offspring retained traits of the parents
 Combined
o Plant breeding
o Statistics
o Careful recordkeeping
 Austrian monk
 Developed the laws of inheritance
 Presented his findings in the Natural History Society of Brunn – 1865
 Paper entitled, “Experiments in Plant Hybridization” – 1866
 German language
 Mendel's work was not recognized until the turn of the 20th century
 1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm
 Called the “Father of Genetics“
Mendel’s Pea Plant Experiments
 Why peas, Pisum sativum?
o Can be grown in a small area
o Produce lots of offspring
o Produce pure plants when allowed to self-pollinate
o Several generations
o Can be artificially cross-pollinated
o Bisexual.
o Many traits known.
o Above all, easy to grow
 Reproduction in Flowering Plants
o Pollen contains sperm produced by the stamen
o Ovary contains eggs found inside the flower
o Pollen carries sperm to the eggs for fertilization
 Self-fertilization can occur in the same flower
 Cross-fertilization can occur between flowers
Mendel’s Experimental Methods
 Mendel hand-pollinated flowers using a paintbrush
 He could snip (cut) the stamens to prevent self-pollination
 He traced traits through the several generations
How Mendel Began?
 Mendel produced pure strains by allowing the plants to self-pollinate for several generations
 True Breeding Plants
o Plants which consistently have offspring with same trait as parent are true breeding plants

Eight Pea Plant Traits
o Seed shape --o Seed Color ---o Pod Shape --o Pod Color --o Seed Coat Color --o Flower position--o Plant Height --o Flower color ---
Round (R) or
Yellow (Y) or
Smooth (S) or
Green (G) or
Gray (G) or
Axial (A) or
Tall (T) or
Purple (P) or
Wrinkled (r)
Green (y)
Wrinkled (s)
Yellow (g)
White (g)
Terminal (a)
Short (t)
White (p)
Monohybrid Cross
 What happens when true breeding plants with two distinct forms of a trait are crossed?
 Progeny show only one form of the trait
 The observed trait is called DOMINANT
 The masked trait is called RECESSIVE
Test Cross
 Is a plant showing the dominant trait true-breeding or not?
 Test by crossing with a plant showing the RECESSIVE trait!!


All Tall offspring indicate parent is true-breeding
Mixed offspring indicate parent is hybrid
Crossing Hybrids to Each Other
 Hybrid parents show the dominant trait (tall)
 Offspring…
o dominant trait (tall) and true-breeding (1/4 total)
o dominant trait (tall) and NOT true-breeding (1/2 total)
o recessive trait (short) and always true-breeding (1/4 total)
 Mendel concluded that among the hybrid parents the short trait (recessive) was hidden but not absent
Mendel’s Data…
 Crossed true-breeding plants differing at one of seven traits
 Crossed hybrid offspring to each other (all show dominant trait)
 Counted offspring of hybrid crosses
Did the observed ratio match the theoretical ratio?
 The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round : 1 wrinkled
 Mendel’s observed ratio was 2.96 : 1
 The discrepancy is due to statistical error
 The larger the sample the more nearly the results approximate to the theoretical ratio
Generation “Gap”
 Parental P1 Generation = the parental generation in a breeding experiment.
 F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)
o From breeding individuals from the P1 generation
 F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation)
o From breeding individuals from the F1 generation
Law of Segregation
 Why do traits “disappear” in one generation only to reappear in a subsequent generation?
 Each plant possesses two distinct separable units (alleles) for each trait inherited from each parent
 Gametes contain ONE allele for each trait
 Only one version is observed in an individual
o The unit (allele) does not disappear
o It may be present but hidden
Alleles
 Mendel’s units (or “elementen”) are called ALLELES
o Versions of the same gene or DNA sequence
o Differ in DNA sequence at one or more sites
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
 Monohybrid cross - cross involving a single trait (e.g. flower color)
 Dihybrid cross - cross involving two traits (e.g. flower color & plant height)
 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 (what you see) (e.g. red, white)
Genotypes
 Homozygous genotype – When the two alleles are same (dominant or 2 recessive genes) e.g. TT or tt; also called pure
 Heterozygous genotype – When the 2 alleles are different- one dominant & one recessive allele (e.g. Tt); also called hybrid
Wildtype
 Most common version in the general population
 Wildtype phenotype (most common phenotype)
 Mutant phenotype (phenotype different from the wildtype)
 Wildtype allele (most frequent allele association with the common phenotype
 Mutant allele (allele associated with the mutant phenotype)
Law of Segregation: Monohybrid Cross



Two heterozygous parents produce gametes with T or t allele equally frequently
Offspring genotypes = ¼ TT : ½ Tt : ¼ tt
Offspring phenotypes = ¾ tall : ¼ short
Modes of Inheritance
 Indicates the patterns with which the mutant phenotype is associated
o Autosomal recessive
o Autosomal dominant
o X-linked recessive
o X-linked dominant
o Y-linked
o Mitochondrial
Autosomal dominant inheritance
 Heterozygotes exhibit the affected phenotype.
 Males and females are equally affected and may transmit the trait.
 Affected phenotype does not skip generation.
Comparison of autosomal dominant and autosomal recessive inheritance
 Autosomal Dominant
o Affects both males and females
o Both males and females transmit the trait
o The trait does NOT skip generations
o At least one parent of affected child must be affected
 Autosomal Recessive
o Affects both males and females
o Both males and females transmit the trait
o The trait DOES skip generations
o Both parents of affected child must be affected
Law of independent assortment
 Two genes on different chromosomes segregate their alleles independently.
 The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.
Independent assortment of two traits
 In a dihybrid cross, parents with two differing traits are crossed.
 Which allele is dominant?



Heterozygous peas are round and yellow
Therefore round is dominant to wrinkled and yellow is dominant to green
Two traits segregating independently
Probability
 The likelihood that an event will occur.
o No chance of event
probability = 0
o Event always occurs
probability = 1
 The probabilities of all the possible events add up to 1
Independent events
 The probability of independent events is calculated by multiplying the probability of each event.
 In two rolls of a die, the chance of rolling the number 3 twice
o Probability of rolling 3 with the first die
= 1/6
o Probability of rolling 3 with the second die = 1/6
o Probability of rolling 3 twice
= 1/6 x 1/6 = 1/36
Dependent events
 The probability of dependent events is calculated by adding the probability of each event.
 In one roll of a die, what is the probability of rolling either the number 5 or an even number?
o Probability of rolling the number 5
= 1/6
o Probability of rolling an even number
= 3/6
o Probability of rolling 5 or an even number = 1/6 + 3/6 = 4/6
Independent events
 What is the chance of an offspring having the homozygous recessive genotype when both parents are doubly heterozygous?
Dependent events
 Parents are heterozygous for a trait, R.
 What is the chance that their child is carries at least one dominant R allele?
o Probability of child carrying RR = ¼
o Probability of child carrying Rr
=½
o Probability of child carrying R_
=¼+½=¾
Punnett Square
 Used to help solve genetics problems
Equation
 The formula 2n can be used, where “n” = the number of heterozygous traits.
 Ex: TtRr, n=2
o 22 or 4 different kinds of gametes are possible
o TR, tR, Tr, tr
Dihybrid Cross
 TtRr X TtRr
 Each parent can produce 4 types of gametes.
o TR, Tr, tR, tr
 Cross is a 4 X 4 with 16 possible offspring.
 RESULTS
o 9 Tall, Red flowered
o 3 Tall, white flowered
o 3 short, Red flowered
o 1 short, white flowered
o Or: 9:3:3:1
Genes and Environment Determine Characteristics
Monohybrid Crosses
P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR
x
rr
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL ALIKE
F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds x Round seeds
Rr
x
Rr
F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
…And Now the Test Cross
 Mendel then crossed a pure & a hybrid from his F2 generation
 This is known as an F2 or test cross
 There are two possible testcrosses:
o Homozygous dominant x Hybrid
o Homozygous recessive x Hybrid
F2 Monohybrid Cross (1st)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds x Round seeds
RR
x
Rr
F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the offspring have SAME genotype as parents
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
Question:
How many gametes will be produced for the following allele arrangements?
Remember:
2n (n = # of heterozygotes)
1.
RrYy
2.
AaBbCCDd
3.
MmNnOoPPQQRrssTtQq
Summary of Mendel’s laws
Summary of Mendel’s Hypothesis
 Genes can have alternate versions called alleles.
 Each offspring inherits two alleles, one from each parent.
 If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent.
 The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation
 The elegance of Mendel’s experiments was partly due to the complete consistency between his observation and Hypotheses he
developed.
 However, after mendel’s work was rediscovered, it became clear that simple medelian model did not adequately predict
experimental observations in all situations.
Pedigrees
 Symbolic representations of family relationships and inheritance of a trait
Autosomal dominant inheritance of brachydactyly
 Heterozygotes exhibit the phenotype.
Autosomal recessive inheritance of albinism
 Heterozygotes carry the recessive allele but exhibit the wildtype phenotype
Genetic predictions
 Ellen’s brother Michael has sickle cell anemia, an autosomal recessive disease
 What is the chance that Ellen’s child has sickle cell anemia allele (a)?