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B2 Genetics: chromosomes; genes; DNA;
recessive; heterozygous; death;
phenotype; homozygous; genotype; allele;
Mendel (1822-1884)
Mendel studied inheritance in peas.
His idea was that organisms
passed on separate
characteristics via “inherited
factors” (we now call genes).
He recognised that some “inherited
factors” were dominant, whilst others
were recessive.
The importance of Mendel’s work was not
recognised until after his death because:
1. DNA, genes and chromosomes had not
been discovered. (Chromosomes were
first seen under a microscope in about
1900. The idea of genes being small
sections of chromosomes which could
be inherited then followed).
2. People struggled to understand his
theories.
Genetic crosses key terms (HT only)
Homozygous = When an organism has two identical copies
of the same allele. For example TT or tt.
Heterozygous =When an organism has two different
alleles of the same gene. For example Tt.
Phenotype = The appearance or characteristics caused by
a particular allele.
Genotype = The combination of alleles found in the cell.
For example TT, Tt or tt.
Family trees: These can be used to track the
inheritance of genetic diseases over several
generations.
If the disease shown is caused by a dominant allele, what are
the genotypes of parents 1 and 2 (use the letters A and a
for alleles)? State clearly if they are homozygous or
heterozygous.
1 = Aa, heterozygous.
2 = aa, homozygous.
Parents
B2 Genetic disorders: carriers, lungs, recessive, two,
dominant, screening, mucus, phenotype, faulty, fingers,
polydactyly, termination, one, toes, healthy, (letters and
outcomes of crosses not included)
Genetic disorders
Genetic disorders can be inherited. They are caused
by faulty alleles. The presence of disease causing
alleles can be confirmed by embryo screening.
Parents can then make informed decisions about
termination of a pregnancy.
Polydactyly
This is caused by a dominant allele so can be
passed on by only one parent who has the
disorder. Sufferers have extra fingers or toes.
Show the outcome of a cross between two parents, one of
which is healthy, whilst the other has one copy of the
disease causing allele.
Let d = healthy allele, D = polydactyly allele
Parents phenotype: healthy
x
Parents genotype:
Offspring:
x
dd
polydactyly
Dd
gametes
d
d
D
Dd (polydactyly)
Dd
(polydactyly)
d
dd (healthy)
dd (healthy)
1 healthy : 1 polydactyly
½ (50%) chance of healthy baby, ½ (50%) chance of
baby with polydactyly.
Cystic fibrosis
This is caused by a recessive allele, so sufferers
need to inherit two copies, one from each
parent. The parents may be suffers of the
disease or symptomless carriers.
The disorder affects the cell
membranes, especially in the
lungs, causing a build up of sticky mucus.
Show the outcome of a cross between two parents, both
of which are healthy, but carriers of cystic fibrosis.
Let F = healthy allele, f = cystic fibrosis allele
Parents phenotype: healthy x
healthy
(carrier)
(carrier)
Parents genotype:
Ff
x
Ff
Offspring:
gametes
F
f
F
FF (healthy)
Ff (carrier)
f
Ff (carrier)
ff (cystic
fibrosis
3 healthy : 1 cystic fibrosis
¾ (75%) chance of healthy baby, ¼ (25%) chance of
baby with cystic fibrosis.
B2 Genetics terms, sex chromosomes, genetic code and genetic fingerprinting: related, 12, XX, protein, gene, father,
fingerprinting, dominant, alleles, recessive, XY, females, unique, 16, 23, amino, sex, males. (No outcome values listed).
Sex chromosomes
In human body cells one of the 23 pairs of
chromosomes carries the genes that
determine sex. In females the sex
chromosomes are the same (XX), in males
they are different (XY).
Parents phenotype:
male
x
female
XY
x
XX
Parents genotype:
Offspring:
gametes
X
X
XX (female)
XX (female)
XY (male)
XY (male)
Genetic terms
Gene
= small section of DNA.
Alleles
= different forms of the same gene.
Dominant = an allele that controls a characteristic
even if present on only one chromosome.
Recessive = an allele that controls a characteristic
only if the dominant allele is absent.
Genetic fingerprinting: Each person (except identical
twins) has unique DNA. Genetic fingerprinting is used
to identify people. Related people share some bands
eg the child and Man B share bands 10 + 16, 12 + 20
so Man B is likely to be the child’s father.
X
Y
Ratio 1 male : 1 female
½ (50%) chance of male baby, ½ (50%)
chance of female baby.
Genetic code (higher tier)
Each gene codes for a particular combination of amino
acids which makes a specific protein.