Std.8 Genetics Review
Summarize Mendel’s laws of genetics. Create and analyze
a punnett square to demonstrate these laws.
Explain how phenotype is related to genotype. Distinguish between
homozygous and heterozygous. (terms: alleles, dominant, recessive)

_________________ alleles need only 1 allele to be expressed; ____________________ alleles must have two alleles to be
expressed.

Define genotype: ____________________________________________________________________________

Define phenotype: ___________________________________________________________________________

Identify sex chromosomes for female: ______________ ;
sex chromosomes for male: _____________
Question Analysis
If a corn plant has a genotype of Ttyy, what are the possible
genetic combinations that could be present in a single grain of
pollen from this plant? (show your work)
The following genotypes were found in a male cat and a female
cat. BbSs (male)
bbSS (female)
Which one of the following choices is true of the phenotype of
offspring from these parents?
a.
All offspring will have black fur.
b.
All offspring will have white fur.
c.
All offspring will have long-haird fur.
d.
offspring will have short-haired fur.
Based only on the sex chromosomes in typical human egg and
sperm cells at fertilization, what is the probability of producing a
female? Prove your answer with a punnett square.
In fruit flies, the gene for red eyes (R) is dominant and the gene
for sepia eyes (r) is recessive. What are the possible combination
of genes in the offspring of two red-eyed heterozygous flies (Rr)
In certain breeds of dogs, deafness is due to a recessive allele (d)
of a particular gene, and normal hearing is due to its dominant
allele (D). What percentage of the offspring of a normal
heterozygous (Dd) dog and a deaf dog (dd) would be expected to
have normal hearing?
A genetic disorder due to a recessive allele (a) is lethal in
homozygous individuals (aa), whereas heterozygous individuals
(Aa) have no symptoms. Based on this information, which of the
following is likely to result?
a.
The disorder will quickly be eliminated since no recessive
homozygotes will survive to reproduce.
b. The disorder will be maintained in the population through the
reproduction of heterozygotes.
c. Only homozygous dominant (AA) individuals will survive.
d. The prevalence of the disorder will increase over time.
Why?
Std.8 Genetics Study Guide
I. Genetics
* Gregor Mendel - studied plants & discovered principles of
inheritance → rules of genetics
 Dominance – can be complete, incomplete or
codominant
 Segregation – alleles segregate (separate) during
gamete formation → meiosis
 Independent assortment – alleles line up and separate
independently during gamete formation.
 Alleles – single form of a gene; come in pairs (Bb, Tt)
II. Genes and dominance
 P-generation = parents (parental generation)
 F1 generation = first filial generation → offspring
 F2 generation = second filial→ second generation
*Dominant alleles are expressed over recessive alleles (masked)
*Allele pairs segregate (separate) during gamete formation



Dominant traits are expressed with a capital letter (T)
(R) (S) while recessive alleles are expressed with lower
case letters (t) (r) (s).
Phenotype – physical characteristics (color, height)
Genotype – genes/alleles → BB, Tt, Ss
2 recessive alleles must be present in order for the trait to be
expressed phenotypically.
III. Dihybrid Crosses
 Crosses involving 2 traits at once (height & color)
 Foil parents genotype to find gametes
o TtRr  gametes = TR, Tr, tR, tr
 Standard dihybrid cross F2 generation  9:3:3:1 ratio.
IV. Human Chromosomes

Humans = 46 chromosomes
o 2 sex chromosomes → X and Y
o 44 autosomes
* sex – male (XY) & female (XX) is determined by father

Karyotype → photos of chromosomes cut & grouped
together; shows abnormalities → extra/missing chroms.
* Human genome is made of thousands of genes
V. Genetic Disorders
* Many genetic disorders are caused by recessive alleles
* A small change in DNA (mutation) of a single gene can affect
the structure of a protein causing a serious genetic disorder



Nondisjunction = failure of homologous chromosomes
to separate during meiosis (Down’s Syndrome)
Sex chromosome disorders – result from nondisjunction
of sex chromosomes
o Turners Syndrome – XO in females
o Kleinfelter’s Syndrome – XXY in males
Sex-Linked Genes –
o Genes located on the X or Y chromosome
o Many genes are located on the X-chromosome
* Since males only have a single X-chromosome, all X-linked
traits will be expressed even if recessive
o Examples = color blindness & hemophilia