Genetic and Inheritance Notes
●​ Gregor Mendel (1822-1884) - The “Father of Genetics” carried out the first scientific
study of heredity between 1857 and 1856.
○​ He used the rules of probability to analyze his results (the first scientist to use
mathematics to support his conclusions)
○​ Mendel used garden pea plants to study heredity.
○​ Mendel first produced “pure-breeding” strains for each of the seven traits he
was studying
○​ Pure-breeding strains only produce one form of a particular trait.
○​ There 14 pure-bred strains were his “parental” generation or P generation
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○​ Mendel then cross-pollinated two pure-bred plants for a single trait
■​ Ex. He crossed a purple flowering plant by taking the pollen from one
flower and transferring this pollen to the other flower
○​ When a “cross” is made, it means the two individuals combined their gametes
○​ Mendel found that all the offspring of this cross were purple
○​ That is, the white flower trait seemed to disappear
○​ These offspring were termed the “first filial” generation or the F1 generation
○​ The offspring of the cross-pollinations are called hybrids, a mixture of two
different organism strains.
○​ The following happened:
1.​ When the seeds were planted and grow, about three-fourths of the
offspring were purple flowers and about one-fourth were white flowers
2.​ These offspring made up the F2 (second filial) generation
3.​ The appearance of white flowering plants in the F2 generation showed
the factor that determined white flower color was still present in the F1
generation, but somehow masked
4.​ Similar results were found for all seven of the pairs of contrasting traits
●​ Dominant Trait: The trait that is expressed (seen) in the F1 generation when the two
purebred parent strains are crossed
○​ Refers to the gene that masks (hides) the presence of another gene for the
same trait
●​ Recessive Trait: The trait that is hidden in the F1 generation
○​ The gene that is masked by the presence of a dominant gene and is the
alternative characteristic to the dominant characteristic
●​ Today we know that each body cell of an organism had two copies of the gene for
each trait
●​ Alleles - The different forms of the gene controlling a certain trait.
○​ Ex. Tall and short pea plants; round or wrinkled seeds
○​ The two alleles (copies) for the gene for a certain trait may be alike or may be
different
○​ It takes the two alleles together to make a gene
○​ Each allele appears on the pair of homologous chromosomes
●​ Homozygous - The alleles for a certain trait in an organism are the same
○​ Homozygous Dominant - The alleles are both dominant
■​ Ex. If a pea plant is a pure-bred tall plant, both gene alleles would be
for tall (tall-tall)
○​ Homozygous Recessive - The alleles are both recessive
■​ Ex. If a pea plant was a pure-bred short plant, both gene alleles would
be for short (short-short)
●​ Heterozygous: The alleles for a certain trait in an organism are different, one is
dominant and the other is recessive
○​ Ex. The F1 generation for a cross between tall and short pea plants would be
heterozygous (tall-short) - each plant would carry the allele for tall and the
allele for short
○​ Heterozygous: Aa (A: tall, a: short)
●​ A capital letter is used for the allele for a dominant trait
○​ Ex. The allele for tallness is represented by the symbol “T”
●​ A lowercase letter is used for the contrasting recessive allele
○​ The allele for shortness is represented by the symbol “t”
●​ Genotype - The genetic (allele) make-up of an organism
a.​ Ex. The genotype of a homozygous dominant (tall) pea plant = TT
b.​ Ex. The genotype of a homozygous recessive (short) pea plant = tt
c.​ Ex. The genotype of a heterozygous tall pea plant = Tt
●​ Phenotype - The physical trait that an organism develops as the result of its
Genotype
a.​ The phenotypes for TT and Tt = tall pea plants since both possibilities carry
the tall allele (T) which wins out over the recessive allele (t)
b.​ The phenotype for tt = short pea plants
Genetic Organization
Genotype
Phenotype
Homozygous recessive
pp
White flower
Homozygous dominant
PP
Purple flower
Heterozygous (one of each gene type)
Pp
Purple flower
1.​ The Law of Segregation - Genes occur in allele pairs and the alleles are separated
from each other during meiosis and are recombine at fertilization
○​ The alleles of a gene are found on the homologous chromosomes that are
separated during meiosis to form gametes
●​ To explain the numerical results of Mendel’s experiments, you must understand the
laws of probability
1.​ Law of Probability - The principle stating that if there are several possible
events that might happen, and no one of these events is more likely to
happen than any other, then they will happen in equal numbers over a large
number of trials
■​ The laws of probability allow scientists to predict the chances of
occurrence for an organism’s genotype or phenotype
■​ The probability may be written as a decimal, percentage, or a fraction
■​ Flipping a coin:2 outcomes (heads or tails)
■​ If you flip a coin 100 times, about:
●​ 50% = heads
●​ 50% = tails
A.​ A Punnett Square is a helpful diagram that helps predict the probability that certain
traits will be inherited by offspring
1.​ The alleles for the male and female gametes are written on top and the
left-hand side of the square
2.​ Each box contains the genotype of the zygote that forms
B.​ Monohybrid cross - A genetic cross in which only one pair of contrasting traits is
studied
○​ Ex. Two heterozygous tall (Tt) pea plants are crossed that contain the tall (T)
and the short (t) alleles, which are contrasting traits (tall vs short)
●​ The Punnett Square shows the four possible genotype combinations for the offspring
●​ Among a large number of offspring, you would expect genotypes of about:
a.​ ¼ to be TT (pure tall; homozygous dominant)
b.​ ½ to be Tt (hybrid tall; heterozygous)
c.​ ¼ to be tt (pure short; homozygous recessive)
●​ The genotype ratios would be 1:2:1
●​ For phenotypes, with a large number of offspring, you would expect about
a.​ ¾ to be tall )TT+Tt
b.​ ¼ to be short (tt)
c.​ The phenotype ration would be 3:1
●​ Law of Dominance - The principle of genetics stating that when organisms pure for
contrasting traits are crossed, all their offspring will show the dominant trait
○​ If a homozygous recessive short plant is crossed with a heterozygous plant,
½ would be tall, ½ would be short
○​ If a homozygous recessive short plant is crossed with a homozygous
recessive plant, all offspring would be short
●​ Blending of Characteristics
○​ It is rare that there exists a single dominant allele or a single recessive allele
for a trait
○​ Incomplete Dominance: A type of inheritance in which two contrasting alleles
contribute to the individual a trait not exactly like either parent and completely
blends the trait
■​ Ex. When homozygous dominant red snapdragon flowers are crossed
with homozygous recessive white snapdragon flower, the offspring are
pink
●​ Codominance - is a type of inheritance in which two dominant alleles are expressed
at the same time without blending of traits
●​ Sex-linked traits
○​ Traits controlled by a gene found on one of the sex chromosomes
○​ Caused mostly by recessive alleles of certain genes found on either the X
chromosome or the Y chromosome
○​ If a sex-linked trait is found on the X chromosome this trait or disease is more
likely to appear in males than in females
○​ There are relatively few Y sex-linked traits and disorders
●​ Ex/Color Blindness - The individual cannot perceive certain colors, usually red and
green. The condition is more common in males than females, although females may
be “carriers” of the trait.
a.​ Every male receives an X chromosome from his mother and a Y chromosome
from his father
b.​ If the mother is a carrier for color blindness, there is a 50% chance that any
son has will receive the defective allele
c.​ Since the Y chromosome has no gene for color vision when a son inherits the
defective allele from his mother, the allele is expressed and the son will be
colorblind
d.​ A female, however, receives an X chromosome from her mother and an X
from her father
■​ A daughter is a carrier only if she has a defective allele from either her
mother or father. The dominant allele will mask the colorblind allele
■​ She will only be colorblind if she receives a defective allele from both
her mother and father
●​ Pedigree Chart
○​ A diagram that shows the presence or absence of a particular trait in each
member of each generation
○​ These pedigree charts may show patterns of inheritance and which
individuals are carriers of particular alleles
■​ Square = male
■​ Circle = female
■​ Filled in: Affected/have a particular trait
■​ Half filled: Carrier (for a sex-linked trait)