1. Genetics Genetics is the science of heredity
Genetics explains how genes bring about characteristics in living organisms and how those characteristics are transmitted from parents to offspring
Genetics is at the center of all biology because genes of an organism determines all biological processes

2. Genetics
Genes are discrete units of inherited information consisting of a specific nucleotide sequences within molecules of DNA
Each gene provides information related to a particular trait (protein) that the organism exhibits

3. 3

4. Experimental Genetics
The modern science of genetics began in the 1860’s when Gregor Mendel, an Austrian monk studied the principles of genetics by breeding garden peas
available in a wide variety of shapes and colors
cheap and abundant
short generation times with large amounts of offspring

5. Experimental Genetics
Mendel studied 7 characters (heritable features) of pea plants each with its own distinctive trait (variant of that character)
He created true-breeding lines of peas that demonstrated only 1 variation of a particular trait over many generations
for example, a true breeding purple pea plant had only purple flowers, not white

6. Flower color
Purple
White
Flower position
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Stem length
Tall
Dwarf

7. Experimental Genetics
Mendel wanted to see what happened when he crossed true-breeding lines for one trait with true-breeding lines for the opposite trait (purple vs white, round vs wrinkled, tall vs dwarf)
His results led to the establishment of several principles:
Mendel’s Law of Dominance
Mendel’s Law of Segregation
Mendel’s Law of Independent Assortment

8. What happens when you cross a true breeding plant that has purple flowers with a true breeding plant that has white flowers?
100% of the plants following the cross have purple flowers (no plant had white flowers)

9. What happens when you cross these 2nd generation purple flowered plants with one another?
75% of the plants following the cross have purple flowers and 25% of the plants following the cross have white flowers 9

10. Mendel’s Law of Dominance
The white and purple flowers of the pea plants are two versions of a gene for flower color
Alternative versions of a gene are called alleles
Mendel’s law of dominance states that when an organism has 2 different alleles for any given character, 1 allele will dominate over the other
the dominant allele is more common in the population
Dominant alleles are designated with a capital letter, whereas their counterpart recessive allele is designated with the same letter, but lowercase

11. P plants Gametes F1 plants (hybrids) Gametes F2 plants
Genetic makeup (alleles)
P plants PP pp
Gametes
All P
All p
F1 plants
(hybrids)
All Pp
Gametes 1 – 2 P 1 – 2 p
F2 plants P p
Phenotypic ratio
3 purple : 1 white P PP Pp
Genotypic ratio
1 PP : 2 Pp : 1 pp p Pp pp

12. Mendel’s Law of Dominance
For each character, an organism inherits 2 alleles, 1 from each parent
These alleles may be the same or different
An organism that has 2 of the same alleles for a trait is said to be homozygous
An organism that has 2 different alleles for a trait is said to be heterozygous

13. Mendel’s Law of Dominance
If the 2 alleles of an inherited pair differ (heterozygous), then one allele will determine the organism’s appearance over the other, and is called the dominant allele
The other allele has no noticeable effect on the organism’s appearance and is called the recessive allele

14. Mendel’s Law of Segregation
A sperm or egg carries only 1 allele (haploid) for each inherited trait
This is because allele pairs segregate (separate) during gamete formation (meiosis)
When sperm and egg unite during fertilization, they each contribute their own allele, restoring the paired ‘condition’ (diploid) to the offspring

15. P a B P a b 15

16. Mendel’s Law of Independent Assortment
The alleles of a gene pair separate from one another independently of the other alleles of another gene pair during segregation (meiosis)
The origin of any particular allele will be randomly selected from paternal or maternal chromosomes via the process of crossing-over
explains why the shape of a pea (round vs wrinkled) is independent of its color (green vs yellow)
peas can be green and round or yellow and round or green and wrinkled or yellow and wrinkled

17. In this example, yellow and green are 2 traits for the color character (indicated by Y and y, respectively) and round and wrinkled are 2 traits of another character (indicated by R and r, respectively)
RRYY
rryy
Gametes RY ry
RrYy
Sperm 1 – 4 RY 1 4 – rY 1 4 – Ry 1 4 – ry 1 – 4 RY
RRYY
RrYY
RRYy
RrYy 1 4 – rY
RrYY
rrYY
RrYy
rrYy
Yellow
round 1 – 4 Ry 16 –– 9
RRYy
RrYy
RRyy
Rryy
Green
round 16 –– 3 1 – 4 ry
Yellow
wrinkled
RrYy
rrYy
Rryy
rryy 16 –– 3
Green
wrinkled 1 16 ––

18. The Chromosome Basis of Inheritance
Mendel established his principles (laws) of inheritance long before mitosis and meiosis were understood, and longer still before chromosomes were ‘discovered’
The chromosome theory of inheritance states that genes occupy specific loci, or location on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis 18

19. Fertilization among the F1 plants
All round yellow seeds
(RrYy)
F1 generation R y r Y R r r R
Metaphase I
of meiosis
(alternative
arrangements) Y y Y y R r r R
Anaphase I
of meiosis Y y Y y R r r R
Metaphase II
of meiosis Y y Y y
Gametes y Y Y y Y Y y y R R r r r r R R 1 – 4 ry 1 – 4 rY 1 4 – Ry 1 4 – RY
Fertilization among the F1 plants
F2 generation 9 :3 :3 :1 19

20. Genetics Terminology
The complete genetic make-up of an organism is called its genotype
The physical expression of the genotype is its phenotype P a B
Genotype: P a b PP aa Bb
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous

21. Mendel’s laws reflect the rules of probability
Mendel’s strong background in mathematics (and physics and chemistry…) served him well in his studies of inheritance
He knew he needed large sample sizes
The laws of inheritance reflect the probability of an event occurring
The probability of having a girl: 1 in 2
The probability of rolling a 5 on a dice: 1 in 6
The probability of drawing a queen from a deck of cards: 4 in 52 (1 in 13)

22. Probability
An event that is certain to occur has a probability of 1 or 100%
An event that is certain not to occur has a probability of 0 or 0%
When you flip a coin, the probability of getting heads (or tails) is 1 in 2 (50%) every time you toss the coin
independent of previous tosses

23. Segregation and fertilization as chance events
Bb male
Formation of sperm
Bb female 1 – 2 B 1 – 2 b
Formation of eggs B B B b 1 – 2 B 1 – 4 1 – 4 b B b b 1 – 2 b 1 – 4 1 – 4
F2 genotypes

24. Genetic traits may be tracked
Individuals exhibiting a recessive trait would be homozygous recessive (carry 2 copies of the recessive allele)
Individuals exhibiting a dominant trait, however, could be homozygous dominant (carry 2 copies of the dominant allele) or be heterozygous (carry 1 copy of the dominant and 1 copy of the recessive allele)

25. Genotype Genotype F_ ff W_ ww ee E_ Dominant Traits Recessive Traits
Freckles
No freckles W_ ww
Widow’s peak
Straight hairline ee E_
Free earlobe
Attached earlobe

26. Genetic disorders
Genetic disorders may be inherited as a recessive or dominant trait
Most human genetic disorders are recessive; most people who have recessive disorders are born to normal parents who are both heterozygous for the allele controlling the disorder
In this way, the parents are carriers of the recessive allele, but are phenotypically normal

27. Offspring produced by parents who are carriers for a recessive trait
Normal Dd
Normal Dd
Parents D
Sperm d Dd
Normal
(carrier) DD
Normal
Does this mean that deaf parents always have deaf children? D
Eggs Dd
Normal
(carrier) dd
Deaf d
Offspring

28. Three’s a crowd…
Mendel was fortunate in that he chose characters for which there were only 2 alleles
Many genes, however, have more than 2 alleles in the population
More often than not, the inheritance patterns of a particular trait are more complex

29. Incomplete Dominance
In some allele combinations, dominance does not exist
Instead, 2 traits are blended together to form a 3rd trait
In snapdragons when a red flowering plant is crossed with a white flowering plant, some offspring are red, some are white and some are pink

30. Red RR White rr P generation Gametes R r F1 generation Pink Rr Gametes– 2 R 1 – 2 r
Sperm 1 – 2 R 1 – 2 r
F2 generation RR rR 1 – 2 R
Eggs Rr rr 1 – 2 r

31. Incomplete Dominance
In this case, heterozygous individuals exhibit a third phenotype, pink.
The resulting pink flowers are Rr and can produce red, white or pink offspring of their own
In the case of incomplete dominance, the phenotype does reveal the genotype for all traits

32. Multiple alleles Multiple alleles exist for most genes
For example, the ABO blood group in humans involves 3 alleles of a single gene: A, B, and O
An individual can have type A, B, O, or AB blood
The A and B alleles are co-dominant; both alleles are expressed in heterozygous individuals

33. Blood
Group
(Phenotype)
Genotypes
Red Blood Cells O OO
AO or AA A
Carbohydrate A
BO or BB B
Carbohydrate B AB AB

34. Multiple alleles
No matter how many alleles for a given gene are in a population, a diploid individual will only have 2 alleles, one on each homologous chromosome
Homozygous
for the
dominant allele
recessive allele
Heterozygous
Genotype: P B a PP aa b Bb