Chapter 15: Mendelian Genetics: Talkin’ ’Bout the Generations summary

Mendel’s laws

  • The dominance law states that one trait can hide another form of the trait. The visible trait is known as the dominant trait while the hidden trait is referred to as the recessive.
  • The segregation law states that when adults make gametes, they give one message to each trait of gamete.
  • The independent assortment law states that every genetic message pair sorts itself independently from other genetic messages.

Importance of Mendel’s works

  • Mendel developed pure-breeding lines in plants. He mated plants with the desired trait of interest to study and ended up with plants containing the version of the trait desired.
  • Mendel preserved numerical records that were detailed and analyzed the crosses from his experiment using statistics. By analyzing crosses, Mendel discovered inheritance patterns that led to understanding inheritance.

Genetic cross of round pea and wrinkled pea

  • Parental plants are pure-breeding hence they possess one allele for the trait.
  • Every pea has 2 alleles for each trait.
  • Each pea donates one allele to its offspring.
  • Round peas allele is dominant to the wrinkled peas allele.

Notations in genetics

  • The parental generation is termed as P1.
  • The first generation results from two parents are referred to as F1 generation.
  • The second-generation results from two F1 cross matches are referred to as F2 generation.

Making predictions

  • Punnet squares enable easier outcome prediction of a certain genetic experiment.
  • A punnet square makes the outcome prediction of F1 self-cross generation easier.
  • F1 generation members are heterozygous for one trait hence called monohybrid cross.

Rules of probability in inheritance

  • The multiplication rule indicates that if 2 events are independent of each other, the probability of them occurring at the same time is derived from a product of their probabilities. This rule can be used to determine the cross outcome.
  • The addition rule states that during the occurrence of an event in more than one way,  then the probability of occurrence is derived from the sum of independent probability for each way of occurrence.


  • Geneticists use pedigrees from families expressing a certain trait to study human traits.
  • Inheritance patterns can also be studied in pedigrees.
  • There are various symbols for human pedigrees:
    • Females are shown by circles and males by squares.
    • A mating is indicated by a drawn line between a male and a female.
    • The offsprings that result from mating are drawn below parents.
    • The symbol for an individual with trait under study is shaded.
Key patterns in pedigree interpretation
  • Traits passed by recessive alleles may skip generations.
  • Traits passed by dominant alleles come up in every generation.
Determining the type of inheritance using pedigree
  • Decide on the inheritance of interest.
  • Genotypes can be assigned to certain individuals.
  • Work from known genotypes of individuals.
  • Continue assigning genotypes using the pedigree.

Puzzling over the punnet

  • Dihybrids are individuals that have two heterozygous traits.
  • The mating individuals from the F1  generation is called a dihybrid cross.
  • The role of a punnet square in a dihybrid cross includes the following:
    • Identification of the types of gametes to be produced by individuals being mated.
    • Design a punnet square with sufficient columns and rows for the types of gametes to be produced by individuals to be mated.
    • Record the types of gametes produced by every parent on the sides and top of the punnet square.
    • Calculate any possible offsprings by combining gametes from Punnett square box.
    • Identify the possible phenotypes from the matching.


Why did Mendel start will purebrid plants for his experiment?
Mendel began with purebred plants because he had to know all of the other recessive traits that weren’t obviously dominant. These plants were absolute copies of each other.

What does the Law of Segregation say?
The Law of Segregation explains during the gamete formation you only get one of the two alleles from each parent.

How did Mendel see that traits are inherited as discrete units?
Mendel saw different colored flowers in the generation F₁ and F₂ and this made him see that traits are inherited as discrete units because there are recessive traits that aren’t visible like the dominant.

How are that terms gene, locus and allele related?
We all have one allele from each parent and on the allele, the location of a gene is called the locus.

Why can an organisms genotype be heterozygous recessive?
You can’t have one allele of dominant and one of recessive and call it heterozygous recessive… in other words it doesn’t exist.

What is one example of a genotype and a phenotype?
genotype- PP-homozygous dominant
phenotype-purple flowers

What was Mendel known as?
father of genetics

How many genes did Mendel use for his genetic experiment?
-pea color
-pea shape
-pod shape
-pod color
-flower position
-plant height

How many genes are included in a monohybrid cross?dyhybrid?

  1. 2.

Describe dominant traits.
They’re that traits that we can see on the outside, these are the ones that are expressed.

Describe recessive traits.
These traits are expressed only when there are two of them.

What is the same thin as purebred?

What is the Pgeneration?
parent generation

What alleles are included in a heterozygous combination?
two different alleles

What alleles are included in a homozygous combination?
two same alleles

Describe genotype and phenotype?
-genetic makeup

  • genes alleles

-physical characteristic words

  • we can see these
  • brown eyes, brown hair

What do the squares in the Punnett Square represent?
They represent that total possible number of allele combinations.

List all possible genotypes of a purple flower(PP)h round seeds(Rr)
PR, PR, Pr, Pr

List all 4 possible allele combinations in gametes of a plant with genotype:PpWw
PW, pW,PW, pw

probability= ?

describe Punnett squares
allow you to predict the genotypic frequency outcomes

monohybrid cross
crosses one trait and come out with three different possibilities

What are the three different possibilities that come out of a monohybrid cross?

when you cross an organism’s unknown genotype with its recessive phenotype

dihybrid cross
crossing two different traits

Law of Independent Assortment
When you have two or more traits, that are going to independently assort with each other

How do you find probability?

of ways an event can occur/total possible outcomes

pertaining to an autosome (first 22 pairs of chromosomes)

sex-linked genes
It is a gene that is located on a sex chromosome

An individual that seems to have one copy of a recessive autosomal allele that causes disease in the homozygous condition

incomplete dominance
A kind of dominance occurring in heterozygotes in which the dominant allele which is partially expressed and normally results in an intermediate phenotype

A condition in which the alleles in a gene pair of a heterozygote are fully expressed thereby resulting in an offspring with a phenotype that is neither dominant nor recessive.

polygenic inheritance
A non-Mendelian form of inheritance in which a particular trait is produced by the interaction of many genes

The interaction between nonallelic genes at two or more loci resulting in one gene masking the phenotypic expression of another gene

What does the Chromosome Theory of Inheritance tell you?
1-genes are located on chromosomes
2- the behavior during meiosis and fertilization accounts for inheritance patterns

A diagram showing lineage or genealogy of an individual and all the direct ancestors, usually to analyze or follow the inheritance of a trait.

to arrange and classify the chromosome complement of an organism based on the number, size, shape and other characteristics of the chromosomes

Resulting to a condition wherein the daughter cells have an abnormal number of chromosomes; one cell having too many chromosomes while other cell having none.

crossing over
the exchange of chromosome segments between homologous chromosomes during prophase 1 of meiosis 1

genetic linkage
genes located close together tend to be inherited together

describe dominant
allele that is expressed when 2 different alleles or 2 dominant alleles are present

allele that is expressed only when 2 copies are present

refers to the genetic makeup of a specific set of genes

the physical characteristics, or traits, of an individual organism

incomplete dominance=?
intermediate inheritance

How are autosomal traits related to Mendel’s experiment?
Autosomal traits are related to Mendel’s observation of heredity because you have two have two copies of autosomal genes to affect a phenotype and that’s what Mendel ended up doing his experiment on.

Describe how sex-linked genes are expressed differently in males and females.
Sex-linked genes are expressed in females when she gets both alleles from parents and a boy only needs it from the mom because it’s homozygous recessive on the X-chromosome.

Is it possible that a son will get that a son will get hemophilia from a homozygous recessive (non-hemophilic)mom and a heterozygous dominant (has hemophilia)?
No, it’s not possible that the male offspring will get hemophilia because it is an X chromosome sex-linked genetic disorder and since mom is the only parent giving the X chromosome, it’s not possible due to the fact that she is homozygous recessive which means she is non-hemophilic.

List all 4 variations of Classical Mendelian Genetics.
1- Incomplete Dominant (Intermediate Inheritance)
2- Codominance (Multiple Alleles)
3-Polygenic Inheritance

Give the definition and an example of Incomplete Dominance (Intermediate Inheritance)
definition-An organism that doesn’t have a dominant or recessive trait.
example- Chickens with black or white feathers when the traits are both dominant or both recessive, the chickens turn blue because neither of the traits override the other trait.

Give the definition and an example of Codominance-Multiple Alleles
definition-both alleles are expressed for a trait
example- blood type

Blood type AB is the only type of codominant blood type-it showcases antigens for blood type A and B

Give the definition and an example of Polygenic Inheritance
definition- traits that are inherited that are controlled in more than two genes
example-eye color, height, skin color, hair color

Give the definition and an example of Epistasis
definition- There are two or more genes but one overrides the other gene(s)
example- Mice
c-no pigment
B-more pigment
b-less pigment

cc-albino no matter what B allele is present

List 3 types of environment that affects you.
build-go work out/exercise
shade of skin- skin’s exposure to the sun

What chromosomes are autosomal?

Which chromosome is the sex chromosome?

what is the genotype of a:


How do males get sex-linked genetic disorders?
They only get x chromosome sex-linked recessive disorders from mom.

How do females get sex-linked genetic disorders?
They get x-linked disorders from mom and dad because it’s recessive so both alleles have to be present for the phenotype to be effected.

List 3 examples of X-linked recessive sex-linked genetics disorders
2-Color Blindness

X chromosome inactivation
some genes are turned on and some are turned off.

What is the Chromosome Theory of Inheritance?
The chromosomal theory of inheritance holds that the separation of maternal and paternal chromosomes during gamete formation is the physical basis of Mendelian inheritance

What is the purpose of a pedigree
using squares representing males and circles representing females, it shows the occurance of a genetic trait in many familial generations

What is a mutation.
A change in an organisms DNA


How is incomplete dominance expressed in a phenotype?
Neither parent’s phenotype is shown but a third phenotype is.

If two parents with O blood type, what type of blood do you expect their first born to have?
Their first born should have O blood type because O blood’s genotype is “ii.” Since the genotype is homozygous recessive, the offspring will need a recessive allele from both parents in order for it to effect the phenotype but the only traits the parents have are for type O blood.

Why do polygenic traits vary in phenotype more than single-gene traits?
Polygenic Traits are controlled by two or more genes so those combinations are variated more than a single-gene trait in which is controlled by one gene.

Why is codominance the same thing as having no dominant or recessive relationship at all between two alleles?
They over ride each other, so, both alleles are expressed.

About how many known genes are found on the X chromosome?
1100 genes

What does a circle always represent in a pedigree?

What does a square always represent in a pedigree?

In a pedigree, what does a shaded circle or square always represent?
That particular offspring or parent has the trait or disease trying to be tracked using the pedigree.

What genotypes can a person with autosomal dominant disease have?
homozygous dominant or heterozygous

entire chromosome layout of someone

During meiosis, chromosome split apart during cell division and nondisjunction causes the chromosomes to stick together, therefore resulting in an abnormal chromosomal count in each cell offspring.

In a pedigree, what does a half shaded circle or square always mean?
That particular offspring or parent is a carrier for a gene, trait, disease or disorder.

What are all the 4 possibilities of a monohybrid cross of a genotype RrYy

Genetic Linkage

  1. the closer two genes are, the more likely they’re to be inherited together
  2. the farther away two genes are on a chromosome from each other, the more likely they will be separated by crossing over

causes sticky chromosomes which leads to an abnormal count of chromosomes in a cell during meiosis

What’s an example of nondisjunction?
Tri-some 21 (Down Syndrome)

Damaged Chromosomes
Part of a chromosome is deleted, some genes are flipped around

What are the three types of mutations and what are they?
chromosomal-chromosomes don’t align or genes aren’t in the correct places on the chromosomes
point-same as substitution for a gene
frame shift- insertion or deletion of a gene

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