1. Mendel’s Laws of Heredity
GENETICS
Mendel’s Laws of Heredity

2. Some Needed Vocabulary
Genetics
The study of Heredity
Heredity
Passing on of characteristics from parent to offspring
Traits
Characteristics that are inherited
Ex: hair color; eye color; height; hairline shape
Gene
Parts of the DNA that code for traits
Allele
Different forms of a gene

3. Gregor Mendel A monk who studied heredity using garden pea plants
Why garden peas?
They reproduce sexually
They have both male & female gametes on the same plant
What did he do?
He transferred pollen from one plant to another with different traits
This is called “making a cross”
Mendel first “crossed” tall plants with short plants

4. Mendel’s Monohybrid crosses
Focuses on one (mono) trait
1st generation
Crossed a tall pea plant with a short pea plant
All of the offspring were tall
2nd generation
Crossing the offspring of the 1st generation
¾ of the offspring were tall; ¼ of the offspring were short
3:1 ratio (tall to short)

5. Mendel’s Findings
Mendel found that each chromosome has two factors for each traits… called alleles
Alleles = forms of genes
Types of alleles
Dominant
The trait that has the ability to mask the other
Represented with a capital letter (A)
Recessive
The trait that will be masked by the dominant trait… it will be there, but will not be expressed
Represented with a lowercase letter (a)

6. Mendel’s Rules Principle of Dominance:
Some alleles are dominant and some are recessive.

7. Mendel’s Rules: Principle of segregation Law of independent assortment
The two alleles for each trait must separate when gametes are formed
A parent will pass down AT RANDOM, only one allele for each trait
Law of independent assortment
Genes for different traits are inherited independently of one another

8. Phenotype vs. Genotype Phenotype Genotype
The way that an offspring looks; how the trait appears; it is ALWAYS a word.
Tall, short
Genotype
The offspring’s genetic combination
TT, Tt, tt

9. Homozygous vs. Heterozygous
Homozygous (“homo” means “same”)
Having two identical alleles for a trait
TT, tt
Heterozygous (“hetero” means “different”)
Having two different alleles for a trait Tt

10. Punnett square
Short way to find the expected proportions of possible genotypes in the offspring of a testcross

11. Mendel’s Punnett Square 1st generation
T = Tall
t = short
TT x tt T T t T t T t t T t T t

12. Interpreting the Punnett Square
All offspring are heterozygous tall (Tt)
Ratio
4:0 (Tall : Short)
Percentage
100% Tall
Each offspring box represents 25% of the population of offspring produced
Tall = T
Short = t T t

13. Mendel’s Punnett Square 2nd generation
Tall = ____
Short = ___
Parents: _______ x _______
Homozygous Tall:
Genotype: _______
How many? ______
Heterozygous Tall:
Short:
TOTAL # OF TALL: _____
TOTAL # OF SHORT: _____
Ratio of Tall : Short  __________
Percentages:
Percent Tall: ___________
Percent Short: __________ T TT 1 t Tt 2 Tt Tt tt 1 T t 3 1 T TT Tt
3:1 t Tt tt
75%
25%

14. A little review: What is genetics? What is an allele?
What does the Principle of Dominance say?
What does the Principle of Independent Assortment state?

15. A little review:
TT?
tt?
Tt?
What is a phenotype?
What is a genotype?

16. A little review
What is the expected phenotypic ratio for the following cross?
B=brown eyes
b=blue
Bb X Bb
What is the genotypic ratio?

17. Patterns of Heredity “Beyond Mendel”
GENETICS
Patterns of Heredity
“Beyond Mendel”

18. Simple Mendelian Inheritance
The type of inheritance we have discussed thus far (Mendel’s peas)
Not all patterns in heredity are as simple…
There is more than just dominant and recessive

19. Incomplete Dominance
The phenotype of the heterozygote is intermediate (BLENDED) between those of the two homozygotes
For Example:
A cross between a particular Red flower and a White flower will produce a Pink flower

20. Punnett Square for Incomplete Dominance
Cross:
(RR) Red flower X (WW) White flower
100% PINK FLOWERS (RR')
RR = Red
WW = White
RW = Pink R R W R W R W W R W R W

21. Codominance
The phenotypes of both homozygotes is produced (SPOTTED) in the heterozygous individual
For Example:
Chick-fil-A cows!
They have BOTH black AND white hairs.

22. Multiple Alleles For Example:
Traits controlled by more than 2 alleles …
For Example:
Multiple alleles are commonly seen in the coat color of rabbits
Also… Blood Typing…

23. Multiple Alleles – Blood Types
Human blood types are determined by the presence or absence of certain molecules on the surfaces of red blood cells
There are three alleles for blood type…
IA…………………… written as “A”
IB …………………… written as “B”
i …………………… written as “O”

24. Multiple Alleles
There are many possible allele combinations that can result in different blood types:
IA IA and IA i = Type A blood
IB IB and IBi = Type B blood
Only one combination results Type AB and Type O blood:
Type O blood : ii
Type AB blood : IA IB

25. Punnet Squares with Multiple Alleles
Set them up like usual:
IA IA x IB IB IA IA
IAIB IB
IAIB IB
IAIB
IAIB

26. Sex-linked traits
Traits that are controlled by genes located on sex chromosomes
These traits help explain why some characteristics and disorders are more common in one sex than in the other
Commonly referred to as “X-linked” because the trait is usually linked to the X-chromosome

27. Background Information
Humans have 46 chromosomes; or 23 pairs of chromosomes
Of these, 22 pairs are autosomes (self chromosomes)
The last (23rd) pair is different in males & females… these are the sex chromosomes
Females have two “X” chromosomes (XX)
Males have one “X” and one “Y” chromosome (XY)

28. Heterozygous Blue-eyed female (XBXb)
“Sex-linked” traits
Green-eyed male (XbY) X
Heterozygous Blue-eyed female (XBXb)
B = Blue eyes
b = Green eyes Xb Y XB
XBXb
XBY Xb
XbXb
XbY

29. Heterozygous Blue-eyed female
“Sex-linked” traits
OFFSPRING
MALES (XY)
Blue Eyes
Genotype _______
How many? _____
Green Eyes
FEMALES (XX)
B = Blue eyes
b = Green eyes
Green-eyed male X
Heterozygous Blue-eyed female Xb Y XB
XBXb
XbXb
XbY
XBY
XBY 1
XbY 1
XBXb 1
XbXb 1

30. Sex-linked percentages
B = Blue eyes
b = Green eyes
Green-eyed male (XbY) X
Heterozygous Blue-eyed female (XBXb) Xb Y XB
XBXb
XbXb
XbY
XBY
Percentages
When figuring percents for sex-linked punnett squares, you must be careful to only count what is asked for…
% of females with blue eyes
% of males with blue eyes
% of females with green eyes
% of males with green eyes
50%
50%
Females
Males
50%
50%