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AP BIOLOGY 11. Genetics: Chapter 14 & 15 Pg. 247 - 286. Mendel’s Laws. Gregor Mendel is credited with discovering the basic principles of genetics These principles or Laws have seen many modifications but basic genetics begins here. Mendelian genetics. Character

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Genetics: Chapter 14 & 15

Pg. 247 - 286

Mendel’s Laws

  • Gregor Mendel is credited with discovering the basic principles of genetics

  • These principles or Laws have seen many modifications but basic genetics begins here.

Mendelian genetics

  • Character

    (heritable feature, i.e., fur color)

  • Trait

    (variant for a character, i.e., brown)

  • True-bred (all offspring of same homozygous variety)

  • Hybridization

    (crossing of 2 different true-breds)

  • P generation

    (true-breeding parents)

  • F1 generation

    (first filial generation - hybrid)

  • F2 generation

    (result of self-pollination from F1)

Genetic vocabulary…….

  • Allele: one alternative form of a given gene pair; more than two alleles can exist for any specific gene, but only two of them will be found within any individual

  • Allelic pair: a combination of two alleles

  • Punnett square: grid to predict the results of a genetic cross between individuals of known genotype

  • Homozygous: pair of identical alleles for a character

  • Heterozygous: two different alleles for a gene

  • Phenotype: the appearance (expression) of a trait in an individual

  • Genotype: specific allelic combination for a certain gene or set of genes

  • Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

Mendel’s Laws

Dominance and Recessive

  • The ability of one allele (the dominant) to either express its phenotype at the expense of an alternate allele, or mask the effect of the other (recessive) member of the pair.

  • Generally the dominant allele will make a gene product (e.g. protein) that the recessive cannot

  • Dominant is not a reflection of the abundance in a population

Example: yellow peas dominant over green; round pea is dominant over wrinkled

Note: The dominant recessive interaction can be

interpreted differently depending on your level of

study as - a whole organism, a bio-chemical level

or a molecular level E.g. Tay-Sachs disease pg. 256

Mendel’s Principles of Heredity

Law of Independence Assortment

During meiosis, the different homologous chromosome assort independently from other homologous chromosomes

Law of Segregation

The alleles for each character segregate (separate) during meiosis I. (homologous pairs separate)

Only one of the allelic pair are in each sex cell

Important Genetic Terms

Monohybrid cross

Cross between parents that differ at a single allelic pair (usually AA x aa)

Dihybrid Cross

Cross between parents that differ at two allelic pairs (AaGg X AaGg)

Mendel’s Laws

Law of Independent Assortment

Chromosomal Basis of Mendel’s Laws

Mendel’s Laws

  • Using a Punnett Square, there are 16 posssible offspring with a phenotype ratio of 9:3:3:1

  • Segregation of gene pairs

  • Independent assortment of unlinked genes,

  • and the Fertilization of an egg by a sperm

    Are All Random Events……

  • Random events occur independently of each other and are unaffected by previous events.


  • Mendelian inheritance reflects the rules of probability.

  • If we know the genotypes of parents, we can predict the genetics of the offspring by using the Laws of Probability


  • Probability events occur in a range from 0 to 1

  • 1 means that an event is 100% certain to occur.

  • 0 means and event is 0% certain to occur.

  • The probability of all possible outcomes in an event must total 1 (100%)

  • Ex. Coin toss, throw of a die


1. Rule of Multiplication

  • The probability that independent events will occur simultaneously (i.e. in a specific combination) is equal to the product (x) of their individual probabilities.

  • Ex: If two parents are Pp for a trait, what’s the probability of them having a homozygous recessive offspring? (ovum and sperm must carry the p allele)


  • Probability of a “p” from egg = 1/2

  • Probability of a “p” from sperm = 1/2

  • Overall probability of two “p’s” uniting at fertilization:

  • 1/2 x 1/2 = 1/4 (.25)

    Problem: If two parents are YyRr, what is probability of them conceiving a yyrr offspring?


2. Rule of Addition

  • The probability of an event that can occur in two or more independent ways is the sum (+) of the the separate probabilities of the different ways.

  • Ex: if two parents are Pp, what is the probability of them producing a heterozygous offspring?


  • There are two independent ways to make a heterozygous offspring: the dominant allele could come from the mother and the recessive from the father, or vice versa.

    The probability of the dominant from mother and recessive from father is: 1/2 x 1/2 = ¼

    The probability of the dominant from father and recessive from mother is: 1/2 x 1/2 = 1/4

    Therefore, the overall probability is the sum of these two probabilities: 1/4 + 1/4 = ½

    Answer: The probability of them producing a heterozygous offspring is a 1 in 2 chance or a 50% chance



1. What is the probability of rolling two 6’s from two dice at the same time?

2. What is the probability of getting a heads - tails combination when tossing two coins simultaneously?

3. Two parents are AaBbCc - what is their probability for a aabbcc offspring?

Mendel’s Laws

Practice Problems:

1. Pigeons can be either plain or checkered color. A series of crosses were made, and the following ratios were observed:

Cross 1: Plain x Plain = All Plain;

Cross 2: Checkered x Checkered = All Checkered;

Cross 3: Checkered x Plain = 1 Checkered:1 Plain;

Cross 4: Checkered x Checkered = 3 Checkered:1 Plain.

Which phenotype is dominant? Create your own gene symbol and give the genotypes of the parents (as close as you can determine) for each cross.

Mendel’s Laws

2. In humans, the allele which confers the ability to roll the tongue is dominant over the allele which does not confer the ability to roll the tongue. If a man who can roll his tongue and whose mother could not mates with a woman who cannot roll her tongue, what proportion of the children would be expected to be able to roll their tongues if they have a large number of children? What are the genotypes which are possible among the children?

Mendel’s Laws

3. In humans, the gene for dimples is recessive to its allele which produces no dimples. Also, the gene for 5 digits per hand and foot is recessive to its allele for 6 digits per hand and foot. Cross a man who does not have dimples, who has 6 digits per hand and foot, heterozygous for both these traits with a women who has dimples and 5 digits per hand and foot. What phenotypes might be present in their children? What is the probability at conception for each phenotype occurring?

Mendel’s Laws

4. A student did a series of genetic crosses on pea plants examining tallness and seed color. What type of cross would produce the following offspring genetic ratios?

a) 3:1

b) 1:1

c) 1:2:1

d) 9:3:3:1

e) 1:1:1:1

f) 2:1

Extending Mendel’s Genetics

  • We have learned much since Mendel’s work with garden peas. We now know that there are many exceptions to “Mendel’s Laws”.

  • Mendel was fortunate that he chose peas and that the seven traits he studied all happened to be on different chromosome pairs.

Non-single gene genetics

  • Incomplete dominance:

    Both alleles are expressed resulting in a blending of traits

    Example: snapdragons can be red or white, but in the heterozygous condition, they are pink

  • Codominance:

    This occurs when both members of an allelic pair are expressed - not blended.

    Example: blood types - Type A and Type B are co-dominant. The heterozygous condition is Type AB

    Note: Dominance ranges from complete dominance, through to various degrees of incomplete dominance, to codominance

Extending Mendel’s Genetics

Multiple Alleles

  • Mendel only ever considered two possible alleles for a trait, ex. Tall (T) or Short (t).

  • In many traits there are more than two possible alleles.

  • For example, in human blood types there are three possible allele: A, B, or O

Extending Mendel’s Genetics


  • This is when one gene or allele affects more than one trait.

  • Ex: Yellow mice are heterozygous; gray homozygous recessive. The homozygous dominant doesn’t exist - it’s lethal. The gene for color also affects viability

  • Ex: sickle-cell anemia.

  • Incomplete dominance –organism

  • Codominance at the molecular level

  • Heterozygous form is beneficial to

  • ward of malaria pg. 262

More Than One Gene Involved

Ex: Chicken combs are controlled by two genes producing four possible phenotypes (dihybrid cross

Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. The interaction between two or more genes to control a single phenotype Ex: mice coat color

Rose Pea

Single Walnut

Extending Mendel’s Genetics

  • Polygenic Inheritance:

  • An additive effect of two or more genes on a single phenotypic character Ex: Skin color and height are controlled by a series of genes which effects multiple depending on each genes dominance

9 B_D_ (black)

3 B_dd (dilute black)

3 bbD_ (brown)

1 bbdd (dilute brown)

Extending Mendel’s Genetics

Sex Determination

  • Unlike most traits, sex is determined by a pair of chromosomes

  • Females have two “X” chromosomes while males have an”X” and a “Y”.

Extending Mendel’s Genetics

Sex Linkage (pg. 277-279)

  • Fathers= pass X-linked alleles to all daughters only (but not to sons)

  • Mothers= pass X-linked alleles to both sons & daughters

  • X-inactivation: 2nd X chromosome in females condenses into a Barr body

  • Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia

  • Males have only one X chromosome so express sex linked genes even if recessive; females show normal inheritance (have two alleles for the trait).

The white eyed fruit fly is a male that inherited a single sex linked mutant gene for eye color. The red eye male is normal.

More practice problems:

  • In fruit flies, eye color is sex-linked. In females, the allele for red eyes is dominant over the allele for white eyes. List phenotypes for the offspring of each of the following crosses and the expected ratio of each phenotype.

    a. A male with red eyes crossed with a female with white eyes

    b. A heterozygous female crossed with a male with red eyes

    c. A heterozygous female crossed with a male with white eyes

    d. A homozygous red-eyed female crossed with a male with

    white eyes

More practice problems:

2. A couple have a son with sex-linked muscular dystrophy.

a. From which parent did the son receive gene(s) for muscular dystrophy?

b. If their next child is a girl, what is the probability she will inherit sex-linked muscular dystrophy?

c. If their next child is a boy, what is the probability he will inherit sex-linked muscular dystrophy?

d. What is the probability of them having three sons in a row all with muscular dystrophy?

e. This couple is expecting yet another child (after the three in d), what is the probability of this child having muscular dystrophy?

Chromosomal errors

  • Nondisjunction:

    The pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II

  • Aneuploidy: chromosome number is abnormal• Monosomy~ missing chromosome• Trisomy~ extra chromosome (Down syndrome)• Polyploidy~ extra sets of chromosomes

Extending Mendel’s Genetics

Sex Nondisjunction

  • Occurs when the egg or sperm have an abnormal number of X or Y chromosomes - baby gets abnormal number

  • Ex. Kleinfelters XXY & Turners, _X


  • The generation time for humans is very long ( 20 - 30 years).

  • Further, we produce relatively few offspring

  • As a result, Mendelian inheritance is difficult to study in us.

  • One useful method is the development and analysis of human family histories in diagrams called Pedigrees.


What can you decipher

From the pedigree?

When boy III-1 (outlined in blue) died suddenly at a football game at the age of 19, his mother II-2, brother and sisters, friends and doctors were confused. An autopsy showed that the young athelete had died from familial hypertrophic cardiomyopathy (HCM), an inherited disease of the heart muscle. condition.

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