1. Mendel & Heredity

2. Genes are segments on the chromosomes that are responsible for inherited traits such as eye color, hair color, skin color, height, etc.
Now we’re going to learn
how these traits are in-
herited by the study of
genetics.

3. I. Genetics: The branch of biology that studies heredity.
Heredity: The passing of characteristics from parents to offspring.
Mendel, an Austrian monk, carried out the 1st important studies of heredity.
The “father” of genetics.
He is credited with stating the
principle that traits among off-
spring result from combinations
of dominant and recessive genes.
Traits :Characteristics that
are inherited.

4. Monohybrid Cross: A cross involving 1 pair of contrasting traits.
Mendel’s 1st experiments are called monohybrid crosses because the 2 parent plants differed by a single trait—height.
Example: crossing a tall pea plant with a short pea plant.

5. Mendel carried out his experiments in three steps:
1. Mendel allowed each variety of garden pea to self-pollinate for several generations. This method ensured that each variety was true-breeding (purebred) for a particular trait; that is all the offspring would show only one form of a particular trait.
Self-pollination-occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or another flower of the same plant.
For example: All the tall pea plants had to produce many generations of nothing but tall plants. All the short pea plants had to produce many generations of nothing but short plants.
These true-breeding plants served as parental generation in Mendel’s experiments.
Parental generation = P1 generation (or P generation)

6. 2. Mendel then cross-pollinated two P1 generation plants that had contrasting forms of a trait, such as a tall and a short pea plant. Mendel called the offspring of the P1 generation the 1st filial generation, or F1 generation. He then examined each F1 plant and recorded the number of F1 plants expressing each trait.
Offspring of the P1
generation = 1st Filial
or F1 generation
Cross-pollination-
involves flower of
two separate plants.

7. 3. Finally, Mendel allowed the F1 generation to self-pollinate
3. Finally, Mendel allowed the F1 generation to self-pollinate. He called the offspring the F1 generation plants and the 2nd filial generation, of F2 generation. Again, each F2 plant was characterized and counted.

8. P1 = 1st generationTall x short
F1=2nd generation= Tall Tall Tall Tall
F2=3rd generation=Tall Tall Tall short
**The “x” means crossed. Mendel transferred pollen from one plant to another plant.
**All of the f generation is tall. It’s as if the shorter parent had never existed.
** ¾ (75%) of the F2 generation is Tall & ¼ (25%) of the F2 generation is short. A ratio of 3:1.
*It was as if the short (recessive) trait had reappeared from nowhere!
*A dominant trait appears in every generation of the offspring. A recessive trait does not.

10. III. Mendel’s Theories of Heredity—The foundation of genetics.
1. Mendel is known as the father/founder of genetics.
2. For each inherited trait, an individual has two copies of the gene—one from each parent.
3. Alleles are different gene forms (different versions). An individual receives one allele from each parent.
Each allele can be passed on when the individual reproduces.
4. 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.

12. Mendel’s 3rd Theory Cont.
Mendel described the expressed form of the trait as dominant. And the trait not expressed when the dominant form of the trait was present was described as recessive
Dominant: The expressed form of the trait. (The trait is visible).
Observable trait of an organism that masks a recessive form of the train.
Dominant genes (alleles) are represented by capital letters and the same letter in small case represents recessive genes. The dominant allele is always written first.
Ex. If tallness is dominant over short pea plants. Tt would be used if the pea plant is tall.

13. Blonde Hair
VS.
Brown Hair

14. Mendel’s 3rd Theory Cont.
Recessive: The trait that is not expressed when the dominant form of the trait is present.
The hidden trait of an organism that is masked by a dominant trait.
For every pair of contrasting forms of a trait that Mendel studied, the allele for one form of the trait was always dominant and the allele for the other form of the trait was always recessive.
Example: Each of Mendel’s pea plants had 2 alleles that determined its height.
A plant could have: 2 alleles for tallness, TT
2 alleles for shortness, tt
1 for tall and 1 for short, Tt

16. 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 fertilization, each gamete contributes one allele. Each parent can contribute only one of the alleles because of the way gametes are produced during the process of meiosis (haploid). + =
Sperm (1n)
Egg (1n)
Zygote (2n)

18. Phenotype: The way an organism looks
Phenotype: The way an organism looks. It is the physical appearance of a trait.
Example: The phenotype of a tall plant is tall regardless of the genes it contains.
Genotype: The gene combination on organism contains (set of alleles). Genotype describes the genetic make up of a trait. The genotype of a tall plant that has 2 alleles for tallness is TT.
**You can’t always know an organism’s genotype simply by looking at its phenotype.

19. Homozygous:. Two alleles for a trait are the same
Homozygous: Two alleles for a trait are the same. (The presence of two identical alleles for a trait.)
Example—The true breeding tall plant that has two alleles for tallness (TT) would be homozygous for the trait height.
Because tallness is dominant, a TT individual is homozygous dominant for that trait.
A short plant would have 2 alleles for shortness (tt); it would always be homozygous recessive for the trait height.

20. Heterozygous:. (hybrid) Two alleles for a trait are different
Heterozygous: (hybrid) Two alleles for a trait are different. (The presence of two different alleles for a trait.)
Example: The tall plant that has one allele for tallness and one allele for shortness (Tt) is heterozygous for trait height.

21. Mendel’s hypotheses brilliantly predicted the results of his crosses and also accounted for the ratios he observed. Because of their importance, Mendel’s ideas are often referred to as the laws of heredity
Law of Dominance:
The dominant gene hides the recessive gene; the recessive gene only shows when the dominant gene is NOT present.
Law of Segregation:
The first law of heredity describes the behavior of chromosomes during meiosis when homologous chromosomes, and then chromatids are separated.
States that the two alleles for a trait segregate (separate) when gametes are formed.

23. Punnett Squares: It is a diagram that predicts the possible offspring of crosses.
We solve genetic problems and predict possible offspring by using the Punnett Squares.
The simplest is four boxes inside a square.
The possible gamete that one parent can produce is written along the top.
The possible gamete that the other parent can produce is written along the left side.
Each box inside the square is filled with two letters obtained by combining the allele along the top of the box with the allele along the side of the box.
The letters in the boxes are the possible genotypes of the offspring.

25. Monohybrid Cross: A cross involving one pair of contrasting traits.
Example #1: Cross a heterozygous tall plant with a homozygous short plant. T=dominant for tall and t=recessive for short. Tt Tt T tt tt t
Possible offspring:
Genotype: 2 Tt; 2 tt
Phenotype: 2 Tall; 2 short
Genotype ratio _2:2______________
Phenotype ratio _2:2_____________
_50__ % tall
_50__ % short Tt tt X

26. T t Tt TT T
Example #2: Cross a heterozygous tall plant with a heterozygous tall plant. Tt tt t
Possible offspring:
Genotype: 1TT, 2Tt; 1tt
Phenotype: 3 Tall; 1 short
Genotype ratio _1:2:1______________
Phenotype ratio _3:1_____________
_75__ % tall
_25__ % short Tt Tt X

27. t t Tt Tt T
Example #3: A homozygous tall plant is crossed with a homozygous short plant. Tt Tt T
Possible offspring:
Genotype: 4Tt
Phenotype: 4 Tall
Genotype ratio _4:0______________
Phenotype ratio _4:0_____________
_100__ % tall
_0__ % short TT tt X

28. Example #4: A homozygous tall plant bred to a heterozygous tall plant.Tt TT T
Example #4: A homozygous tall plant bred to a heterozygous tall plant. TT Tt T
Possible offspring:
Genotype: 2TT;2Tt
Phenotype: 4 Tall
Genotype ratio _2:2______________
Phenotype ratio _4:0_____________
_100__ % tall
_0__ % short TT Tt X

29. s s
Example #5: A dog that is heterozygous for curly hair is crossed with a dog that is homozygous recessive for straight hair.
ss= straight hair
SS=wavy hair
Ss=curly hair Ss Ss S ss ss s
Possible offspring:
Genotype: 2Ss;2ss
Phenotype: 2 curly; 2 straight
Genotype ratio _1:1______________
Phenotype ratio _1:1_____________
_50__ % tall
_50__ % short Ss ss X

30. Probabilities Can Also Predict
The Expected Results of
Crosses:
Probability—is the likelyhood
that a specific event will occur.
Consider the possibility that
a coin tossed into air will land
on heads (1 possible outcome.)
The total number of all possible outcome is two—heads or tails. Thus, the probability that a coin will land on heads is ½ or 50%.
Probability = number of 1 kind of possible outcome
total number of all possible outcomes

31. Example : Assume that in humans there is a 50/50 chance that a child will be a boy. If a certain mother and father have four sons, what are the chances that their fifth child will be a daughter. X X XX XX X XY XY Y
number of 1 kind of possible outcome OR 1
total number of all possible outcomes XY XX X

32. Traits Influenced by the Environment:
Environmental Influences:
Making inheritance more difficult to understand are interactions between genes and the environment.
The expression of some traits is affected by internal environments that are governed by age or sex.
Expression of other traits is affected by external factors in the environment such as temperature, chemicals, or light.
Example: Environmental influences fur color in the artic fox—in the winter it has white fur; it has dark fur in the summer.

33. Summer
Winter

34. Human Genetic Disorders Caused by Mutations:
Changes in genetic material are called mutations.
The harmful effects produced by inherited mutations are called genetic disorders.
Many mutations are carried by recessive alleles in heterozygous individuals. This means two phenotypically normal people who are heterozygous carriers of a recessive mutation can produce children who are homozygous for the recessive allele.

36. A genetic trait that appears in every generation of offspring is called dominant.
Scientists can determine several pieces of genetic information from a pedigree.
Autosomal trait—if a trait is autosomal, it will appear in both sexes equally.
Autosomal does not involve the sex chromosomes.
Each chromosome carries genes for certain traits.
Recall that in humans, the diploid number of chromosomes is 46 or 23 pairs.
There are 22 pairs of matching homologous chromosomes called autosomes.

37. Autosome is a chromosome other than X or Y sex chromosomes.
The 2 chromosomes in a homologous pair of autosomes look exactly alike.
In a pair of homologous chromosomes, 1 comes from the mother, and one comes from the father.
In other words, out of 46 chromosomes, 23 comes from mom and 23 from dad.
Homologous chromosomes have genes for the same traits arranged in the same order.
However, because there are different possible alleles (versions) for the same gene, the two chromosomes in a homologous pair are not identical to each other.

38. The 23rd pair (sex chromosomes) of chromosomes differs in males and females. These two chromosomes determine the sex of an individual and are called sex chromosomes.
Sex-linked trait— Traits controlled by genes located on sex chromosomes.
In humans, the chromosomes that control the inheritance of sex characteristics are indicated by the letters X and Y.
If you are a female, XX, your 23rd pair of chromosomes look alike.
If you are a male, XY, your 23rd pair of chromosomes look different

39. MOST TRAITS ARE NOT CONTROLLED BY SIMPLE DOMINANT/RECESSIVE ALLELES
If a trait is sex linked, it is usually seen only in males. Most sex-linked traits are recessive.
Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked condition.
A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is dominant allele on her other X chromosome.
She will express the recessive condition only if she inherits two recessive alleles.
MOST TRAITS ARE NOT CONTROLLED BY SIMPLE DOMINANT/RECESSIVE ALLELES
Patterns of heredity Can Be Complex:

40. State Test Question
Humans have 23 pairs of chromosomes. How many pairs are sex chromosomes 1 2 3 4

41. Sickle Cell Anemia
An example of an autosomal recessive gene disorder is sickle cell anemia.
A condition caused by a mutated allele that produces a defective form of the protein hemoglobin.
Sickle cell anemia is caused by a recessive gene on chromosome 11.
In sickle cell anemia, the defective
form of hemoglobin causes many
red blood cells to bend into a sickle
shape.
Normal RBCs last ~120 days in blood
stream, while sickle RBCs last ~ 10-
20 days. Sickle cells tend to get stuck
in small blood vessels and block the
flow of blood.
Sickle cell anemia is a recessive
disease..

42. Sickle Cell Anemia

43. S s
Example: A woman who is a carrier (Ss) for sickle cell has children with a man who is free of the disease (SS). What are the chances of the couple having a child with sickle cell anemia? Ss SS S SS Ss S 0%
________ chance SS Ss X

44. Tay-Sachs Disease
TSD is an extremely rare autosomal recessive genetic disorder. It causes deterioration of mental & physical abilities; destruction of the Central Nervous System, and fatty material builds up in the nerve cells of the brain.
The child appears normal at first. At 6 months, the child becomes blind, deaf, & unable to swallow over time. Muscle begin to atrophy & paralysis sets in; death by age 4.
No treatment or cure; found on chromosome 15.

45. Tay-Sachs Disease

46. Cystic Fibrosis
An autosomal recessive gene disorder found to affect chromosome 7. This disorder causes production of a protein that makes a person’s body produce unusually thick, sticky mucus.
This mucus can lead to severe infections, & it prevents enzymes from the pancreas from helping the body to break down food & absorb nutrients.
This person usually has a persistent cough, frequent lung infections, salty tasting skin, infertility, and weight loss even when he/she or she eats well, & shortness of breath.
There is no known cure; therefore, cystic fibrosis is often fatal. However, medical treatment may help patients live longer.

47. Huntington’s Disease
Huntington’s disease is a genetic disorder caused by a dominant allele located on an autosome.
In time, HD causes loss
of muscle control, un-
controllable physical
spasms, severe mental
illness, and eventually
death.

48. h h
Example: If one parent is heterozygous for the trait and the other has normal alleles, what are their chances of having an offspring with the disease? Hh Hh H hh hh h
50%
________ chance Hh hh X

49. Phenylketonuria (PKU)
A recessive disorder that results from the absence of an enzyme that converts one amino acid, phenylalanine cannot be broken down, it and its by-products accumulate in the body and result in severe damage to the central nervous system and mental retardation.

50. PKU