Inheritance
DNA Basics
Almost all animal, plant and fungal cells have a
nudei
…………………….………..…….…… containing DNA. DNA contains the
proteins
instructions for making all the ……….………………………….. an organism needs, such as enzymes. A length of
gene
DNA which is the code for making a particular protein is called a ……………..………
The word for the all the
genome
genetic instructions in an organism is called the ……………………………….…………….
chromosomes
The DNA is organised into one or more lengths called …………………..……………………………………….
Most human
23 pairs), because there are 2 copies, these cells are called
cells have 46 chromosomes (……..
ploid
di…………..……………
not
Remember, bacterial cells have a single chromosome in a loop which is ………..……
contained in a nucleus.
gametes
Human cells which don’t contain the full set of 46 chromosomes are the ……..…………………………..
(the sex
haploid
cells) which contain half the normal amount – the keyword for this is …………….…………………..
Variation
Use your knowledge or your notes to complete this table about genetic variation:
Word
Allele
Dominant
Recessive
Meaning
is an alternative form of a gene
a dominate allel is an allel that is expressed if it is present in the genotype
a recessive allel is an allel that is only expressed when there is no dominate allel of the
gene present in the genotype
Heterozygous
having two different allels of a particular gene
Homozygous
having two identical alleles of a particular gene
Phenotype
the observe features of an organism
Genotype
Codominance
the genetic makeup of an organism in terms of the allels present
a situation in which both allels in the heterozygons of organisims contirbute tp the phenotype
Genetics Practice Problems
1. For each genotype below, indicate whether it is heterozygous (He) or homozygous (Ho)
ho
AA _____
he
Ee ____
he
Ii _____
he
Mm _____
he
Bb _____
ho
ff ____
he
Jj _____
ho
nn _____
2. For each of the genotypes below describe the phenotype.
Purple flowers are dominant to white flowers.
Brown eyes are dominant to blue eyes
purple
PP __________________
brown
BB ________________
purple
Pp __________________
brown
Bb ________________
white
pp __________________
blue
bb ________________
Round seeds are dominant to wrinkled seeds.
Bobtails in cats are recessive.
round
RR __________________
non bobtail
TT _________________
round
Rr __________________
non bobtail
Tt _________________
wrinkled
rr __________________
bobtail
tt __________________
3. For each phenotype below, list the genotypes (choose an appropriate letter yourself)
Straight hair is dominant to curly.
TT straight
____
Tr straight
____
rr curly
____
Pointed leaves are dominant to round
leaves.
AA pointed
_____
Ab pointed
_____
bb round
_____
Monohybrid crosses
A monohybrid cross is the study of the inheritance of one characteristic. Questions in exams will ask you
use genetic diagrams to describe and explain how a characteristic could be inherited and also to use these
to predict probabilities.
Firstly make sure you show the possible alleles carried
by the parents’ cells (their genotype).
Then make it clear which alleles could be carried by the
gametes, remember that games only carry 1 copy of a
gene / 1 allele.
Use a crossing over diagram like the one to the left, or a
punnet square, to then show the possible genotypes of
the offspring.
RR
dark
Rr
Rr
dark
dark
Finally, state the phenotype that goes with each
genotype.
rr
light
Complete the diagram on the left with the possible
genotypes and phenotypes of the offspring.
Some practice exam questions
Q1. In pigeons, the inheritance of bars at the end of the wings is controlled by a gene with two alleles.
The allele for wing bars (B) is dominant to the allele for no wing bars (b).
The diagram shows the phenotype of a pigeon with wing bars and the phenotype of a pigeon with no
wing bars.
(i) In a first cross, a heterozygous parent mates with a homozygous recessive parent.
Complete the table by giving the genotype of each parent and complete the drawing to show the
possible phenotypes of their offspring.
(2)
Bb
bb
(ii) In a second cross, two heterozygous pigeons mate.
Give the possible genotypes of their offspring.
(1)
(iii) This second cross only produces one offspring.
Give the probability that this offspring will have wing bars.
(1)
33.33%
.........................................................................................
(Total for question = 4 marks)
Q2. In mammals, males have two different sex chromosomes (X and Y) and females have
two similar sex chromosomes (X and X).
However, in birds males have two similar sex chromosomes (Z and Z), while females have two different
sex chromosomes (Z and W).
(a) Complete the genetic diagram to show how sex is inherited in birds.
(4)
Z and Z
Sperm cells
Z and W
egg cells
male or female
(Total for question = 4 marks)
Q3. Achondroplasia is an inherited condition in humans. Adults with achondroplasia are much shorter than
average height.
This condition is controlled by a gene with two alleles. The dominant allele (A) codes for shorter than
average height and the recessive allele (a) codes for average height.
(a) Two parents both had achondroplasia. They had a child who grew up to be of average height.
Use a genetic diagram to show:
•
•
•
•
the genotype of each parent
the gametes they produced
the genotypes of all the possible offspring
the phenotypes of all the possible offspring
(4)
(b) The parents had a second child.
State the probability that this child grew up to be of average height.
(1)
..............................................................................................................................................
(Total for question = 5 marks)
Pedigree Chart Activity
A pedigree is a diagram that shows the occurrence of a particular characteristic, as it is passed from one
generation to the next in a given family. From this information, along with an understanding of inheritance,
genotypes of individuals can often be determined.
Symbols for a Pedigree Chart
Follow the story below and create a pedigree chart.
This is the story of Grandma and Grandpa Fellowes, and their clan!
They were married way back in 1933 and have been just like
newlyweds ever since. From their union, 4 individuals were created.
Elizabeth, the eldest, was born in 1935. Fred soon followed in 1936.
In 1939 Michelle was brought into this world. Mickey, a surprise to
the whole family was the baby of the family, not being born until
1950.
Elizabeth fell in love at a young age, and wed her high school
sweetheart, David, in 1954. From this marriage, two bundles of joy
came about (at the same time): John and Sonny – 1955 (twins!)
It took Fred a little longer to find his soul mate. Finally in 1970, Fred found Wilma, and they were married. Since
they married so late in life, they only brought one new person into this world: Barney – 1972
Michelle sadly died at a young age.
Mickey chose Monica to spend the rest of his life with. They were wed in 1975 and brought two girls into this
world: Krista – 1977 and Janet – 1979
Draw the pedigree chart in the space below
(a) Now that you have your pedigree chart, shade the appropriate circles and squares using the
information below.
Remember that in this case, individuals who possess the RECESSIVE trait are to be filled in completely.
Individuals not shaded in have at least one dominant allele.
Freckles
Not having freckles is a recessive trait.
Grandpa Fellowes did not have freckles (so shade him in), but his beautiful bride did.
Fred and Michelle were the only two of their siblings to have freckles.
Of the grandchildren, the twins did not have freckles, and neither did Barney, but the two girls did.
(b) Now determine the genotypes of all individuals in this pedigree chart for freckles. Remember, having
no freckles is a RECESSIVE trait.
STEP 1
Using the letter B or b, assign two recessive alleles to any person on the pedigree whose symbol is shaded.
STEP 2
Assign one dominant allele to any person on the pedigree whose symbol is unshaded.
STEP 3
Use the information given to you to determine the second alleles for each person with the dominant
phenotype. There is one person for whom it is not possible to work out their full genotype.
HINT: We know that Grandpa Fellowes does not have freckles. If Grandma were BB, could any bb children
be produced from Grandma and Grandpa? If Grandma were Bb, could any bb children be produced from
them? Complete Punnett squares below, if necessary, to help determine your answers.
(c) Label your pedigree with the correct genotypes for each generation.
Q5. Huntington's disease causes damage to the nervous system. It is an inherited condition caused by a
dominant allele (H). Only individuals who are homozygous for the recessive allele (h) are protected from
the disease.
The diagram shows the inheritance of Huntington's disease in a family.
(a) Complete the table to show how many people in the diagram fit each description. The first one has
been done for you.
(4)
(b) Individuals A and B have both male and female children.
Draw a genetic diagram to show how they can produce both male and female children.
(3)
(Total for question = 7 marks)