Mendelian genetics, Punnet square, sex-linkage

1. THEORETICAL
GENETICS

3. Vocabulary preview
 genetics:  homozygous:
 fertilization:  heterozygous:
 true-breeding:  dominant:
 trait:  recessive:
 hybrid:
 gene:
 allele:
 segregation:
 gamete:

4.  genetics: The study of inheritance
 fertilization:
 true-breeding:
 trait: an observable physical characteristic
 hybrid:
 gene: a heritable factor that determines a specific
characteristic
 allele: one of a number of different forms of a
gene
 segregation: separation of the genes
 gamete: sex cells, i.e: ovules and sperm cells

5.  homozygous: having two identical alleles of a
gene
 heterozygous: having two identical alleles of a
gene
 dominant: an allele that has the same effect on
the phenotype whether it is present in the
homozygous or heterozygous state
 recessive: an allele that only has an effect on the
phenotype when it is present in the homozygous
state
 genotype: the alleles of an organism

6. Prior knowledge: traits

10. The work of Gregor Mendel
 In the 19th century, most biologists worked by
observing and describing nature. Gregor
Mendel was one of the first to apply an
experimental approach to the question of
inheritance. His work eventually gave rise to
genetics, the study of heredity.

12. The blending hypothesis of
inheritance
 In the early 1800s, biologists proposed the
blending hypothesis to explain how offspring
inherit traits from both parents.
 For example, a red-flowered plant crossed
with a yellow-flowered plant of the same
species.
 According to the blending hypothesis, the red
and yellow hereditary material in the offspring
would blend, producing orange-flowered
plants—like blending red and yellow paint to
make orange paint

13. Mendel´s plant breeding
experiments
 For seven years, Mendel bred pea plants and
recorded inheritance patterns in the offspring.
 Based on his results, he developed a
particulate hypothesis of inheritance.
 This hypothesis states that parents pass on to
their offspring separate and distinct factors
(today called genes) that are responsible for
inherited traits

16. Mendel´s Principle of
segregation
 Hybrids: the offspring of two different true-
breeding varieties (white flower and purple
flower)
 The parental plants are called the P generation
(P for parental), and the hybrid offspring are
the F1 generation (F for filial).
 When the F1 plants self-fertilize or fertilize
each other, their offspring are the F2
generation

17. Example of a monohybrid cross
Monohybrid: Different in only one characteristic

18. Class activity
 Pick a pea pod
 Open it carefully
 Draw and describe the outside and the inside
of your pod, what did you find?

19. Compare your pod to the chart
If you had to take a guess, would you say your pod is true
breeding or a hybrid? What evidence do you have?

20. Mendel´s hypothesis
 1. There are alternative forms of genes. For
example, the gene for flower color in pea
plants exists in one form for purple and in
another form for white = Alleles
 2. For each inherited character, an organism
has two alleles for the gene controlling that
character, one from each parent. Alleles are
the same = homozygous (AA). Alleles are
different = heterozygous (Aa)

21. Mendel’s hypotheses
 3. When only one of the two different alleles in
a heterozygous individual appears to affect the
trait = dominant allele (Aa). The other allele
that does not appear to affect the trait =
recessive allele (Aa).
 4. The two alleles for a character segregate
during the formation of gametes, so that each
gamete carries only one allele for each
character = Mendel's principle of segregation.

22. Probability and Punnet Squares
 For a monohybrid cross = only one trait:
 P generation: both true breeding: AA x aa
 F1 generation: all hybrid: ½ Aa x ½ Aa
 F2 generation: ¼ AA, ½ Aa, ¼ aa
AA x aa
Aa x Aa
AA Aa Aa aa

23. Probability
 Remember that during fertilization gametes
combine randomly, so Mendel’s hypotheses
represent a probability

24. Hands-on activity
 2 coins, work in pairs
 Predict the outcome of 36 coin tosses.
 Write down the results for each flip
combination: heads/heads, heads/tails,
tails/heads, tails/tails
 Calculate the fraction of the total tosses for
each combination
 Tally up the results for all student pairs
 Calculate the fraction of the tosses for the
class total

25. Punnet’s square: F1 generation
 A punnet’s grid helps to explain the
segregation of alleles and their effect on the
organism’s phenotype
 1. Draw the grid T T
 2. Place the alleles
 3. Fill the squares t T t T t
 4. Compare dominant
and recessive traits t T t T t

26. Punnet’s square: F2 generation
 A punnet’s grid helps to explain the
segregation of alleles and their effect on the
organism’s phenotype
 1. Draw the grid T t
 2. Place the alleles
 3. Fill the squares T T T T t
 4. Compare dominant
and recessive traits t T t t t

27.  Remember dominant alleles are written with
capital letters : T
 Recessive alleles are written with small
letters: t
Phenotype: The traits expressed (seen) are
dictated by the dominant alleles in the case of
heterozygous individuals
Recessive alleles influence the phenotype only
when we have homozygous individuals: tt
Online activity: 10.2

28. Data-based questions: coat color in
the house mouse
 In the early years of the 20th century, many crossing experiments
were done in a similar way to those of Mendel. The French
geneticist Lucien Cuénot used the house mouse, Mus musculus,
to see whether the principles that Mendel had discovered also
operated in animals. He crossed normal grey-colored mice with
albino mice. The hybrid mice that were produced were all grey.
These grey hybrids were crossed together and produced 198 grey
and 72 albino offspring.
 1. Calculate the ratio between grey and albino offspring, showing
your working.
 2. Deduce the color of coat that is due to a recessive allele, with
two reasons for your answer.
 3. Choose suitable symbols for the alleles for grey and albino coat
and list the possible genotypes of mice, using your symbols,
together with the phenotype for each genotype.
 4. Suggest how one gene can determine whether the mice had
grey fur and black eyes or white fur and red eyes.

29. Variations of inheritance
patterns
 Co-dominance: or intermediate inheritance
 There is no dominant-recessive, rather both
dominants: CBCB and CWCW

30.  Both alleles affect the phenotype, however this
does NOT support the blending hypothesis.
 Each allele codes for a particular
characteristic, and both are then shown
physically (like coding for the production for
the production of a protein that gives color)
 The ratio in this case is 1:2:1
 Homozygous dominant: hybrids: homozygous
recessive

31. Multiple alleles
 Some genes have more than one allele.
Example: blood type in humans.
 Blood groups: A, B, AB and O

32.  The four blood types result from various
combinations of 3 alleles, symbolized as IA (for
carbohydrate A), IB (for carbohydrate B), and i
(for neither A nor B).

33.  Note that blood type O is recessive.
 The alleles for carbohydrate A and B are co-
dominant, meaning they are both expressed in the
phenotype (hence the group AB).
 Polygenic inheritance refers to many genes
affecting a single characteristic, such as height
and skin color in humans.
 Influence of the environment: the product of a
genotype is generally not a single, rigidly defined
phenotype, but a range of possibilities influenced
by the environment.

34. Chromosome Theory of
inheritance
 “Genes are located on
chromosomes (locus), and
the behavior of
chromosomes during
meiosis and fertilization
accounts for inheritance
patterns.”

35.  Chromosomes undergo segregation and
independent assortment during meiosis.
 Every diploid organism has two sets of
homologous chromosomes (one from mom and
one from dad)
 The alleles of a gene reside in the same locus on
both homologous chromosomes.
 Crossing over can recombine gene loci on
homologous chromosomes. This is unlikely when
the genes are very close together. A crossover is
more likely to recombine the alleles when the
genes are far apart

37. Genetic linkage and crossing
over
 It’s the tendency for the alleles on one
chromosome to be inherited together.
 The closer two genes are on a chromosome,
the greater the genetic linkage.
 The farther apart the genes are, the more
likely it is that a crossover event will separate
them.

38. Sex-linkage
 Many species have sex chromosomes, X and
Y, that are associated with determining the
organism’s sex.
 Any gene located in a sex chromosome is a
sex-linked gene.
 Discovered by Thomas Hunt Morgan while he
was studying the inheritance of white eye color
in fruit flies.

39. Thomas Hunt Morgan was studying the
inheritance of white eye color in fruit flies.
White eyes are very rare. Normally, fruit
flies have red eyes.
When he mated a white-eyed male fly with
a red-eyed female fly, all the F1 offspring
had red eyes. The allele for red eyes was
dominant. When Morgan bred F1 offspring
with each other, he got the classical 3 : 1
ratio of red-eyed to white-eyed flies in the
F2 generation.
A surprising twist: none of the flies with
white eyes was female. Morgan realized
that in these flies, eye color must

40.  Morgan deduced that the gene involved in this
inheritance pattern is located only on the X
chromosome.
 There is no corresponding eye color locus on
the Y.
 Thus, females (XX) carry two copies of the
gene for this character, while males (XY) carry
XR Xr XR Xr
only one.
XR XRXR XRXr Xr XX XX
Y XRY XrY Y XY XY

41. Hemophilia: a famous example of
sex-linkage
 Sex linkage: The pattern of inheritance where
there are differences in genotypes and
phenotypic ratios between males and females.
 Hemophilia: the ability of the blood to clot is
severely reduced.