This presentation is all about genetics and inheritance. Grades 6-12. Biology 1. Original Author - Megan Jandy

1. M R S J A N D Y
Genetics and Inheritance

2. A Quick Review
 A gene is a section of DNA
that is transcribed and
translated into a single
protein
 Each chromosome has up
to 25,000 genes
 Humans have 46
chromosomes. (23
homologous from mom and
23 homologous from dad)

3. Gregor Mendel
 A monk who began studying pea plants in 1843
 Discovered and described the basic principles of
heredity (how genes are passed from parents to
offspring)
Mmm… peas….

4. Mendel’s Experiments
 Mendel started fertilizing pea
plants by hand
 He realized that if he bred a “tall”
plant with another “tall” plant that
the offspring would all be tall.
 Parents called P1 generation
 Offspring called F1 (fillial)
 He called the parents “purebreds”
as they produced offspring that
looked exactly like them
 Studied seed shape, plant height,
pod color, flower color…
X
P1
F1

5. Mendel’s Experiments
 He then bred pure plants with
different characteristics (green
pod plant with yellow pod plant)
and the offspring (F1) all turned
out green!
 Where did the yellow pods go?
 He called these offpsring hybrids
(offspring produced by breeding two
different pure lines)
 He then bred these F1 plants to
produce an F2 generation
 ¼ of the F2 generation plants had yellow
pods and ¾ had green pods. What gives?
 Clearly he thought something
strange was going on…
F1
F2
X

6. Mendel’s Hypothesis
 Mendel then hypothesized that there are two
possibilities for each trait (green or yellow pods)
 He called the green pods a dominant trait because it was a
more powerful trait that showed up more often
 He called the yellow pods a recessive trait because it
sometimes disappeared and showed up less often
 He then realized that if an offspring had one
dominant (green) and one recessive trait (yellow),
the dominant trait would show up (green pods)
 If the offspring had two recessive traits, the recessive
trait would show up (yellow pods)

7. Mendel’s Hypothesis
 He did the same
experiment except with
plants that had purple
and white flowers and
saw the same pattern!
 Purple = dominant
 White = recessive
 Because the dominant
(purple) trait always
covered up the recessive
(white) trait he called
this complete
dominance

8. Stop, Pause and Think!
 Think about the following…
1)What are sections of DNA that contain heredity
information ?
2) How does a purebred differ from a hybrid?
3) In a cross that displays completed dominance, if
an offspring carries 1 dominant factor and 1
recessive factor, which trait will the offspring have?

9. Stop, Pause and Think!
1) What are sections of DNA that contain heredity
information ?
genes
2) How does a purebred differ from a hybrid?
A purebred the offspring always has the same traits as
the parent
A hybrid is the result of breeding two different
purebreds.
3) In a cross that displays completed dominance, if an
offspring carries 1 dominant factor and 1 recessive factor,
which trait will the offspring have?
The dominant trait

10. Stop, Pause and Think!
 You should be able to define the following
(Write down in notebook)
1) Gene
2) Purebred
3) Hybrid
4) Complete dominance
5) Dominant trait
6) Recessive trait

11. Principal of Segregation
 Each chromosome has 2 copies
of a gene or trait (one on each
chromatid)
 These two chromatids separate,
or segregate during meiosis
when gametes are formed
 Each parent contributes one of
its copies of a trait to its
offspring
 The chances of contributing
either factor are equal (50/50)

12. Alleles
 We now know that the units of heredity
are genes, and the different forms of
the genes are called alleles
 If the offspring has two dominant
alleles, the offspring will appear to
show the dominant trait
 If the offspring has one dominant allele
and one recessive allele, the offspring
will show the dominant trait
 If the offspring has two recessive
alleles, the offspring will show the
negative trait
Green + Green =
Green + Yellow =
Yellow + Yellow =

13. Alleles
 Each individual carries one copy of a gene (allele)
from their mother and one copy of a gene (allele)
from their father
Chromosome #3
from mother
Chromosome #3
from father

14. Representing Genes and Alleles
 Scientists use abbreviations to show dominant and
recessive alleles
 They use the same letter for the dominant and
recessive allele for each trait
 Dominant allele is capitalized: Green pods (G)
 Recessive alleles are lower case: Yellow pods (g)

15. Genotype vs Phenotype
 Genotype: which
copies of the gene
the organism has
 What the genes code
for
 Phenotype: which
trait does the
organism show
 What you see

16. Determining Genotype
 If you know the phenotype can you determine the
genotype?
 If the pea plant has purple flowers: could have one dominant
and one recessive (Pp), or two dominant (PP) alleles
 If the pea plant has white flowers: must have two recessive
(pp) alleles
 If the organism has two of the same allele, the
organism is called homozygous
 PP = homozygous dominant
 pp = homozygous recessive
 If the organism as one of each allele, the organism is
called heterozygous (Pp)

17. Punnett Squares
 A way to visualize test
crosses (breeding two
organisms)
 Can be used to determine
the probability of
genotypes and phenotypes
of offspring

18. Punnett Squares
 Now you try!
? ?
? ?

19. Stop, Pause and Think!
 Think about the following…
1) What does the principle of segregation state?
2) What is an allele?
3) What is the difference between a genotype and a
phenotype?
4) Define homozygous and heterozygous.

20. Stop, Pause and Think!
1) What does the principle of segregation state?
Members of each pair of genes separate when gametes are formed
2) What is an allele?
Different representations of a gene
3) What is the difference between a genotype and a phenotype?
Genotype is the representation of the alleles; ex: BB or bb
Phenotype is the physical representation of the trait; ex: Black or
white
4) Define homozygous and heterozygous.
Homozygous is when both alleles are the same: ex: BB or bb
Heterozygous is when you have one dominant and one recessive
allele; ex: Bb

21. Stop, Pause and Think
 On your vocabulary sheet define the following
1) Allele
2) Genotype
3) Phenotype
4) Homozygous
5) Heterozygous
• In your notes write down what the Principle of
Segregation States

22. Incomplete Dominance
 In the pea plants Mendel studied, 1 allele was clearly
dominant over the other
 However, this is not always the case!
 Some alleles show incomplete dominance (blend
of traits instead of one or the other)
 Ex: red flowers and white flowers make pink flowers
 In incomplete dominance the phenotype of a
homozygous dominant individual will be different
than the phenotype of the heterozygous individual
CRCR = CRCW= CWCW =

23. Demonstrating Incomplete Dominance

24. Co-dominance
 Co-dominance occurs when
both alleles are visible in
the phenotype (but not
mixed like incomplete
dominance!)
 Ex: This Camellia flower is
not pink, instead its petals
have red and white parts

25. Stop, Pause and Think!
 How could you tell the difference between an
organism and its offspring that show complete
dominance and an incomplete dominance?
 What is the difference between incomplete
dominance and co-dominance?

26. Stop, Pause and Think!
 How could you tell the difference between an organism
and its offspring that show complete dominance and an
incomplete dominance?
Organisms with complete dominance will only show two
variations of a trait. Organisms with incomplete dominance
will show three variations of a trait (one mixed)
 What is the difference between incomplete dominance
and co-dominance?
Incomplete dominance is when two traits are mixed (red +
white = pink)
Co-Dominance is when two traits are both expressed (red +
white = part red and part white)

27. Stop, Pause and Think!
 On your vocabulary sheet define the following
1) Incomplete Dominance
2) Co-Dominance

28. Multi-allele Systems
 Some traits are the result of more than 2 alleles at a
locus (location of an allele on a chromosome)
 Ex: ABO blood system
 IA = produces A antigen on blood cell
produces B anti-body in blood serum
 IB = produces B antigen on blood cell
produces A anti-body in blood serum
 i = produces no antigen on blood cell
produces both A and B anti-bodies in blood serum
 IA and IB are co-dominant
 i is recessive

29. The ABO Blood System
Clots when
exposed to
B-antigen
Clots when
exposed to
A-antigen
Does not
clot when
exposed to
antigens
Clots when
exposed to
A or B-
antigens

30. Law of Independent Assortment
 Mendel showed that dominant traits do not always
show up together (don’t always see green pods and
purple flowers in the same plant)
 Law of Independent Assortment: 2 or more
pairs of alleles separate independently during the
formation of gametes
 Ex: equal chances of inheriting blonde hair allele/brown eyes allele
or blonde hair allele/blue eyes allele
 Traits are inherited separately from each other

31. Sex Linkage
 Autosomal trait
o A gene carried on one of the 22 pairs of non-sex
chromosomes
 Sex-linked trait
 A gene carried on one of the pairs of sex-
chromosomes
 If female XX if male XY
 If X chromosome codes for the allele, then
females will have 2 copies of the allele and
males will only have one copy
 Y-linked trait
 Only males (XY) will have a copy of the allele
 Very rare in humans

32. Showing Sex-Linkage
 Symbols are written as superscript of the sex
chromosome:
 Xa - X chromosome carrying the recessive allele
 XA - X chromosome carrying the dominant allele
 X - No superscript is used for the normal (wild type)
allele
 If you see a different ratio of the trait in males and
females its probably a sex-linked trait!

33. Pedigrees
 Pedigrees are used to determine mode of inheritance
when few individuals, but several generations are
involved
 Assume genetic trait discussed is rare, so individuals
marrying into the family are not assumed to carry the
trait
 Symbols:
female not affected male not affected
female affected male affected
female carrier male carrier

34. Pedigree Analysis

35. Stop, Pause and Think!
 On your vocabulary sheet define the following
1) Multi-allele system

36. Polygenic Inheritance
 Most traits are not limited to two
possibilities (green or yellow)
 Most traits are a continuum
(many act together to determine
phenotype)
 Ex: height, skin color
 Polygenetic Inheritance: two
or more genes act additively on a
trait

37. Stop, Pause and Think!
 Think about the following…
1) What is an example of a trait that is the result of
multi-allele systems?
2) Which blood type is homozygous recessive?
3) How would you be able to tell if a disease was a
sex linked trait by looking at a pedigree?

38. Stop, Pause and Think!
1) What is an example of a trait that is the result of
multi-allele systems?
Blood type
2) Which blood type is homozygous recessive?
O blood
3) How would you be able to tell if a disease was a sex linked
trait by looking at a pedigree?
If the trait is present in a higher ratio in one sex when
compared to the other

39. Stop, Pause and Think
 On your vocabulary sheet define the following
1) Multi-allele system
2) Law of Independent Assortment
3) Autosomal trait
4) Sex-linked trait
5) Pedigree
6) Polygenic Inheritance

40. Dihybrid Crosses
 Dihybrid cross involves the cross of two organisms
while looking at two different genes
 Can demonstrate independent assortment
 Cross organism with two homozygous dominant and
two homozygous recessive genotypes
 YYRR x yyrr can produce:
 YYRR and YyRr = yellow round
 YYrr = yellow wrinkled
 yyRR and yyRr = green round
 yyrr = green wrinkled
 Produces these genotypes in a 9:3:3:1 ratio

41. Diagramming a Dihybrid Cross

42. How to do a Dihybrid Cross
 Lets cross two plants one with (R = red flowers) (T =
tall) RrTt and RrTt
 Draw out a 4 x 4 Punnentt square

43. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Start at the top and fill out the first allele of the top
line (R and r)
RRrr

44. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Now do the same thing with the second alleles but
alternating (T and t)
R R r r
T
T
t
t

45. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Now do the same thing with the second set of alleles
RT Rt rT rt
RT
Rt
rT
rt

46. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Starting at the top fill out the first alleles on the whole
Punnett square
R R r r
R R r r
R R r r
R R r r
RT Rt rT rt
RT
Rt
rT
rt

47. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Do the same thing with the first allele on the vertical
axis
RR RR rR rR
RR RR rR rR
Rr Rr rr rr
Rr Rr rr rr
RT Rt rT rt
RT
Rt
rT
rt

48. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Starting at the top fill out the second allele on the
whole punnet square
RRT RRt rRT rRt
RRT RRt rRT rRt
RrT Rrt rrT rrt
RrT Rrt rrT rrt
RT Rt rT rt
RT
Rt
rT
rt

49. How to do a Dihybrid Cross
 Crossing RrTt and RrTt
 Now do the same thing with the second allele on the
vertical axis
RRTT RRtT rRTT rRtT
RRTt RRtt rRTt rRtt
RrTT RrtT rrTT rrtT
RrTt Rrtt rrTt rrtt
RT Rt rT rt
RT
Rt
rT
rt

50. How to do a Dihybrid Cross
 Analyze the data!
 Make a tally of all possible phenotypes
RRTT RRtT rRTT rRtT
RRTt RRtt rRTt rRtt
RrTT RrtT rrTT rrtT
RrTt Rrtt rrTt rrtt
RT Rt rT rt
RT
Rt
rT
rt
Red/Tall – IIII IIII = 9
Red/Short- III = 3
White/Tall- III = 3
White/Short- I = 1
9:3:3:1 Ratio