What were the results of Mendel’s experiments, or crosses?

Chapter 5 Genetics: The Science of Heredity

• What were the results of Mendel’s experiments, or crosses?

• What controls the inheritance of traits in organisms?

Section 1:Mendel’s Work

What is Genetics?

Genetics: the study of heredity

Heredity: the passing of physical characteristics from parents to offspring

The Father of Genetics

The field of Genetics was founded by Gregor Mendel, an Augustinian priest.

Between 1856 and 1863, Mendel cultivated and tested almost 30,000 pea plants.

The importance of Mendel's work was not discovered until almost 30 years after Mendel died.

Crossing Pea Plants

Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this.

Mendel’s Experiments

In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

Dominant and Recessive Alleles

Mendel studied several traits in pea plants.

Today, scientists use the word “gene” to describe a piece of DNA that controls a trait.

The traits that Mendel studied in his pea plant experiments are controlled by different genes:

Seed Shape

Seed color

Seed coat color

Pod shape

Pod color

Flower position

Stem height

These genes usually have 2 or more alleles, or different forms of the gene:

GENE ALLELE ALLELE

Seed Shape round wrinkled

Seed color yellow green

Seed coat color gray white

Pod shape smooth pinched

Pod color green yellow

Flower position side end

Stem height tall short

Some of these alleles are known as dominant. Others are known as recessive:

DOMINANT RECESSIVE

GENE ALLELE ALLELE

Seed Shape round wrinkled

Seed color yellow green

Seed coat color gray white

Pod shape smooth pinched

Pod color green yellow

Flower position side end

Stem height tall short

In a dominant allele, the trait always shows up as long as there is at least one dominant allele.

T = tallt = short

T t“hybrid tall”

T T“pure tall”

In a recessive allele, the trait only shows up if both alleles are recessive.

T = tallt = short

t t“pure short”

Dominant alleles are always symbolized with capital letters. Recessive alleles are always symbolized with lower-case letters.

Key for Height

T = tallt = short

Key for Seed Color

Y = yellow seed colory = green seed color

Key for Pod Color

G = green pod colorg = yellow pod color

Key for Coat Color

A = gray coat colora = white coat color

End of Section: Mendel’s Work

What is probability and how does it help explain the results of genetic crosses?What is meant by genotype and phenotype?What is codominance?

Section 2: Probability and Heredity

A Punnett Square

The diagrams show how to make a Punnett square. In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele.

Probability and Genetics

In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

Phenotypes and Genotypes

An organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or allele combinations.

Practicing Punnett Squares

1) T T x T T

2) t t x t t

3) T t x T t

Key for Height

T = tallt = short

Key for Seed Color

Key for Pod Color

Practicing Punnett Squares

1) Y y x y y

2) Y Y x y y

3) g g x G g

4) G g x G g

Key for Height

T = tallt = short

Key for Seed Color

Key for Pod Color

Homozygous vs. Heterozygous

Homozygous = 2 identical alleles

also called “pure” or “purebred”

Examples: T T t t

Heterozygous = 2 different alleles

also called “hybrid”Examples: T t

Codominance

In codominance, the alleles are neither dominant nor recessive. As a result, both phenotypes are expressed in the offspring.

Incomplete Dominance

In incomplete dominance, the contributions of both alleles are visible and do not overpower each other in the phenotype. As a result, both phenotypes look “mixed”.

Dihybrid Cross

Key for Height

T = tallt = short

Key for Seed Color

Key for Pod Color

Dihybrid Cross

End of Section: Probability and Heredity

What role do chromosomes play in inheritance?What events occur during meiosis?What is the relationship between chromosomes and genes?

Section 3: The Cell and Inheritance

MeiosisDuring meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism.

Punnett Square

A Punnett square is actually a way to show the events that occur at meiosis.

A Lineup of Genes

Chromosomes are made up of many genes joined together like beads on a string. The chromosomes in a pair may have different alleles for some genes and the same allele for others.

Human Chromosomes

Humans have 23 pairs of chromosomes:

23 from their mother, and 23 from their father.

Human Chromosomes

The first 22 pairs are organized and named according to their size:

Chromosomes #1 are the largest, #2 are the second largest, etc.

Human Chromosomes

The final pair of chromosomes (“X” and “Y”) are the sex chromosomes because they determine the gender of the person:

XX = girl

XY = boy

Sex Chromosomes

Father

The father is who determines the gender of the child since males need a “Y” chromosome, and only males have “Y” chromosomes.

The mother can only give out an “X”, and both boys and girls have at least 1 “X” chromosome.

End of Section: The Cell and Inheritance

• What forms the genetic code?• How does a cell produce proteins?• How can mutations affect an organism?

Section 4: Genes, DNA, and Proteins

The DNA Code

Chromosomes are made of DNA. Each chromosome contains thousands of genes. The sequence of bases in a gene forms a code that tells the cell what protein to produce.

How Cells Make Proteins

During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein.

Mutations

Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism’s trait, or phenotype, may be different from what it normally would have been.

Damages Made by Mutation

THEBIGBADCATATETHEBIGREDRAT

End of Section: Genes, DNA, and Proteins

What were the results of Mendel’s experiments, or crosses?

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