Chapter 11-1 – The Work of Gregor Mendel
Heredity - the passing of traits from parents to their offspring
Genetics – the branch of biology that focuses on heredity
Gregor Mendel, an Austrian monk in the mid-1800’s, published the work that later
became the basis for the study of modern genetics.
- Worked with pea plants because:
a. Has many traits that have two clearly different forms that are easy to tell
apart.
b. Mating of the pea plants can be easily controlled with self-fertilization or
with cross-pollination.
- Self-fertilization – pollen fertilizes the egg cells on the same plant
- Cross-pollination – pollen fertilizes the egg cells on a different plant
- Garden peas are small, easily grown, matures quickly, and produces
many offspring.
c. Patterns established the basis of genetics – the branch of biology that
focuses on heredity.
- Mendel was the first to develop rules that accurately predict patterns of
heredity and used numbers to predict results
- Deemed the Father of Genetics
- Initial experiments were monohybrid crosses – crosses involving only one
set of traits (white and purple flowers, short or tall plants, etc)
- Three steps of experiments;
a. allowed plants to self-pollinate to ensure the plant of being a truebreeding plant for a particular trait.
These true-breeding plants served as the parents or the P generation –
the first two individuals that are crossed in a breeding experiment.
b. Cross-pollinated the true-breeding plants (P generation) with the
offspring called the F1 generation – first filial generation. All were
categorized and counted.
c. Allowed the F1 generation to self-pollinate and called them the F2
generation – second filial generation. All were categorized and counted.
- Results
– all of the F1 plants showed the same traits, the contrasting form of the
trait disappeared.
– In the F2 generations, the missing trait reappeared in some of the
plants.
– In the purple flowers and the white flowers cross, the F1 generation
showed all purple flowers
– In the F2 cross, 705 plants had purple flowers and 224 had white.
This is a ratio of 705:224.
(Divide the larger number by the smaller number to reduce it to the
lowest ratio available)
This works out to about 3 : 1
Chapter 8 – 2 Mendel’s Theory
Before Mendel’s work, most individuals thought of inheritance as a blending of traits. Mendel’s results did not
support the blending hypothesis.
Mendel’s conclusions:
1. for each inherited trait, and individual has two copies of the gene – one form from each parent
2. There are alternative versions of the genes. (alleles – different versions of a gene, one inherited from
each parent).
3. When two different alleles occur together, one of them may be completely expressed, while the other
may have no observable effect on the organism’s appearance.
Dominant – the expressed form of the trait
Recessive – the form of the trait that is not expressed in the presence of the dominant trait
4. When gametes are formed, the alleles for each gene in an individual separate independently of one
another. Thus gametes carry only one allele for each inherited trait. When gametes unite during
fertilizationm each gamete contributes one allele.
Gametes are formed during meiosis and have only half the number of chromosomes for an individual.
In modern terms, Mendel’s findings are noted by the following rules:
Letters are used to represent alleles.
Dominate traits are indicated by using a capital letter (T)
Recessive traits are indicated by using a lower case letter (t)
Two identical alleles present in an individual at the same time makes the individual Homozygous for
that trait (Ex. TT or tt)
When alleles for a particular trait are different (one is dominant and the other is recessive) the individual
is said to be Heterozygous for that trait (Tt or Pp).
NOTE: There are two letters because each individual has two alleles (copies) of a gene.
In heterozygous individuals, only the dominant trait is expressed.
A set of alleles that an individual possesses is called their genotype.
The physical appearance of a trait is called a phenotype.
By convention, the capital letter is always written first.
Mendel’s Laws of Heredity
1. The law of segregation – that the two alleles for a trait segregate (separate) when gametes are
formed.
2. The law of independent assortment – the alleles of different genes separate independently of each
other during gamete formation. This applies only to genes located on different chromosomes or that
are far enough apart on the same chromosome.
To study the law of independent assortment, Mendel conducted a dihybrid cross – a cross that considers
two traits at the same time (ex. Pod color and plant height).
Quiz – Applied Biology
Name _____________________________________
1. Identify the dominant trait and the recessive trait by writing dominant or recessive on the line.
B _________________________
b __________________________
T _________________________
P __________________________
2. Identigy the following pairs of alleles as either heterozygous or homozygous by writing Heterozygous or
Homozygous on the line.
Bb __________________________
PP _________________________
TT __________________________
Tt _________________________
3. If black (B) is dominant to brown (b), what is the phenotype of a mouse that has the genotype of Bb?
_______________________________
4. If purple flowers (P) are dominant to white flowers (p), what is the phenotype of a plant that has a genotype
of PP? ____________________________
5. If freckles (F) are dominant to no freckles (f), what is the phenotype of an individual that is ff?
Quiz – Applied Biology
Name _____________________________________
4. Identify the dominant trait and the recessive trait by writing dominant or recessive on the line.
B _________________________
b __________________________
T _________________________
P __________________________
5. Identigy the following pairs of alleles as either heterozygous or homozygous by writing Heterozygous or
Homozygous on the line.
Bb __________________________
PP _________________________
TT __________________________
Tt _________________________
6. If black (B) is dominant to brown (b), what is the phenotype of a mouse that has the genotype of Bb?
_______________________________
4. If purple flowers (P) are dominant to white flowers (p), what is the phenotype of a plant that has a genotype
of PP? ____________________________
5. If freckles (F) are dominant to no freckles (f), what is the phenotype of an individual that is ff?
Chapter 8 – 3 Studying Heredity
Punnett squares can predict the expected results in crosses.
- Punnett square is a diagram that predicts the expected outcomes of a genetic cross by considering all
possible combinations of gametes in the cross.
- Possible gametes of one parent is listed across the top of a box and the other parent is listed on the
side of the box. The box is then filled in by using the letters at the top and the side of the rows and
columns
- Examples PP x pp, Gg x Gg
- Express ratios of expected crosses as 3:1 or 1:2:1
Test crosses – cross an individual whose phenotype is dominant but genotype is unknown with an individual
whose genotype is homozygous recessive.
Example: A Yellow pea plant (dominant) is crossed with a green pea plant (recessive).
If one half of the respective offspring of the unknown plant expresses the recessive trait, then the
original genotype would be heterozygous.
Probability – the likelihood that a specific event will occur.
- Expressed as a fraction, decimal, percentages, or words.
- If an event will definitely occur, its probability can be expressed as either 1 out of 1, as 1, 100 % or
as 1/1.
- Formula is :
- Probabilty = number of one kind of possible outcome / total number of all possible outcomes
Probability of a specific allele in a gamete – 1 / 2
To find the probabilty that a combination of two independent events will occur, multiply the separate
probabilities of the two events.
Ex. The probability that nickel and a penny will both land on heads is
½ x½ =¼
Pedigrees – a family history that shows how a trait is inherited over several generations.
Carriers – individuals who are heterozygous for an inherited disorder but do not show the symptoms of the
disorder.
Carriers can pass the disorder on to their offspring.
Pedigrees – squares are males
Circles are females
Horizontal lines indicate matings – vertical lines indicate offspring and are arranged in birth order from
left to right.
Autosomal or sex-linked.
Autosomes are chromosomes other than the sex chromosomes.
Sex-linked trait – a trait whose allele is located on the X chromosome. (Most are recessive)
Dominant or Recessive
Autosomal dominant – every individual with the trait will have a parent with the trait
Recessive – the individual with the trait can have one, two or neither parent exhibit the trait.
Heterozygous or Homozygous
Two individuals that are heterozygous carriers of a recessive mutation will not show the mutation, but
may have a child who is homozygous for the trait
Chapter 8 – 4 Patterns of Heredity can be Complex
Traits are influenced by several genes
- polygenic trait – several genes influence a trait
- can be located on the same chromosome or scattered on different chromosomes
- Ex. Eye color, height, weight, and hair and skin color
Intermediate traits –
- Incomplete dominance – a trait that is intermediate between the parents traits
- Ex. Curly hair x straight hair = wavy hair
- Ex. Red petunias x white petunias = pink petunias
-
Codominance – two dominant traits are expressed at the same time
- Ex. Red haired horse x white horse = roan colored horse (has both red and white hairs)
-
Multiple alleles – genes with three or more alleles
- Blood types – ABO ( IA, IB, I )
Traits influenced by the environment
- hydrangeas are either blue or pink - pH
- fur on Siamese cats or Arctic foxes - temperatures
- height, weight, skin color – environmental influences
Mutations – changes in genetic material
- harmful mutations are called genetic disorders
- usually recessive
- sickle cell anemia
- hemophilia
- Tay-Sachs disease
- Cystic Fibrosis
- Huntington’s disease
Detecting and treating genetic disorders
- genetic counseling is medical counseling about whether or not to have children
- gene therapy