Genes and Heredity
BACKGROUND
 It had been known for 1000’s of years that traits were inherited. i.e. that offspring often looked like their
parents.
 Gregor Mendel (1822-1884) was an Austrian monk who experimented with garden peas and developed the
foundation of modern genetics. He noticed that peas had several traits and always showed only one or the
other, rather than a blend (which was previously believed). He crossed plants with different traits to see
what the offspring would look like.
 Mendel found that no matter what combinations he tried, one trait always dominated and masked the other.
In other words, the plant always showed one trait but not the other. It didn’t matter if the trait came
from the male or female parent. The traits were controlled by something he called factors which were
later known as genes.
 Mendel’s Laws of Heredity
1. Inherited traits are controlled by versions of genes that occur in pairs. The two versions are called
alleles. One allele for each trait is inherited from each parent
2. One allele always hides or masks the presence of the other. This is called the principle of
dominance. The dominant and recessive genes are indicated by an upper-case and lower-case letter,
respectively.
3. Alleles are separated during meiosis I. This is called the Law of Segregation.
 Vocabulary
o heterozygous - the alleles for trait are different.
o homozygous - both alleles for a trait are the same.
o dominant - the allele that masks the presence of the other.
o recessive - the allele that is masked by the other.
o genotype - the actual genetic makeup for a trait.
o phenotype - the way in which the genotype is expressed.
o P – parent generation
o F1 and F2 – Latin” filial means “son”. F1 and F2 mean the first and second generations, respectively.
MONOHYBRID CROSS
 Involves one gene pair for one trait.
 There will always be a 3:1 phenotypic ratio in F2 when crossing homozygous dominant and homozygous
recessive parents.
o Example: Seed shape –- Round(R), wrinkled (r)
 Cross a homozygous dominant parent with homozygous recessive parent
P
gametes
F1

Use two of the F1 individuals as new parents to create F2
P
gametes
F2

RR
x
rr
R, R
r, r
Rr (all round)
Rr
x
Rr
R, r
R, r
RR, Rr, Rr, rr (round, round, round, wrinkled; 3:1)
A Punnett square can be used to show genotype, phenotype, and probability of getting a certain phenotype
or genotype.
A
p
p
B
T
t
o Example A: Crossing a heterozygous purple (Pp) x white (pp).
 F1 phenotypic ratio - 1 purple : 1 white
T TT Tt
P Pp Pp
o Example B: Crossing two heterozygous tall plants (Tt).
t
Tt tt
 F1 phenotypic ratio - 3 tall : 1 short
p pp pp

Test cross - the test cross is used to determine the genotype of a particular phenotype. If an organism
has the dominant phenotype, you can’t be sure whether it is homozygous dominant or heterozygous. By
crossing it with a homozygous recessive individual, you can infer the genotype by observing the offspring.
o P
T_ x tt
o F1
all tall; therefore unknown parent must be TT.
o If F1 1 tall :1 short, then unknown parent must be heterozygous.
DIHYBRID CROSS
 Involves two gene pairs for two traits.
 Mendel wondered if traits always traveled together or if they were inherited separately. Ex. Imagine a
cross between a yellow round and green wrinkled parent. Would all the offspring be either yellow round or
green wrinkled or would some be yellow wrinkled and some green round?
 Mendel showed that each trait is inherited independently. Remember the Law of Segregation and
Independent Assortment.
 Example:
o Cross P
YYrr x yyRR – all F1 will be heterozygous for both traits (yellow, round seeds).
o Cross F1
YyRr x YyRr
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRR
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
o
F2
9 yellow, round: 3 yellow, wrinkled: 3 green, round; 1 green, wrinkled.
UNUSUAL INHERITANCE PATTERNS
 Incomplete Dominance
o Before Mendel, it was believed that parental traits were blended in offspring and could not be
separated in later generations. The traits that Mendel studied showed this was not the case, but
this is only true of traits which show so called “simple dominance.”
o With incomplete dominance, two phenotypes are blended rather than showing two different
possibilities. Ex. flower color in snapdragons.
 Codominance - Both of the alleles are expressed at the same time. Ex. roan cattle
 Multiple alleles - For traits discussed so far, there have always been only two alleles in a population. For
some traits, there are more than two alleles. Remember, no matter how many alleles exist, each individual
has only two - one from each parent. Ex. Eye color
 Epistasis - a gene at one location affects the expression of a gene at another location. Ex. coat color in
mice.
 Polygenic inheritance - Some traits show additive effects of two or more genes. Ex. skin color in humans
 Sex-linked inheritance
o A male always inherits a sex-linked trait from the female parent because the male parent always
supplies the y chromosome. Ex. in humans, red-green color-blindness is X-linked.
o Barr body - one X chromosome in each cell of a female becomes inactive during embryonic
development. Ex. Calico cats.
 Linked genes
o Traits that are inherited together. How could this be possible?
o How is it possible for phenotypic variation in the offspring?
HUMAN GENETIC DISORDERS
 Recessively inherited - if a disorder is lethal before reproductive age, the allele frequency decreases
because homozygotes don’t reproduce to pass on the gene. Usually, heterozygotes can survive because
enough of the normal protein is produced.
 Dominantly inherited - the lethal dominant allele is more rare because even heterozygotes die. If the
disorder only affects individuals after reproductive age, the allele will be maintained in the population
because they can pass it on before the disorder is fatal.
 Genetic counseling - for many genetic disorders it is wise for parents who are carriers to be counseled so
they can be prepared to either cope with the disorder or to terminate the pregnancy.
o Carrier recognition - screening prospective parents to determine if they are carriers of a genetic
disorder.
o Fetal testing - methods for testing a fetus in utero to determine if it carries genetic disorders.
 amniocentesis - a small sample of amniotic fluid is withdrawn and the fetal cells it contains
are cultured for a few weeks. The cells can then be tested for genetic disorders. This
procedure can be done by the 14th to 16th week.
 CVS - a sample of the chorionic villi is obtained and the cells are tested for genetic
disorders. The technique can be done by the 8 th to 10th week and results are available in 24
hours.
 fetoscopy - a small camera is inserted into the uterus to visually examine the fetus for
physical defects.
o New-born screening - a neonatal technique whereby the baby is screened for genetic disorders.
 Ethical and moral implications - for many, these techniques are controversial as they could be used as
grounds for terminating a pregnancy. Parents who are not prepared for the challenge of having a child with
a particular disorder can choose abortion.