Chapter 4

Heredity and Genes

4.1 Basic Principles of

Heredity

Heredity

• The passing of traits from one generation to the next.

• Trait: Characteristic, such as eye color or nose shape.

• Genetics: The field of biology related to studying heredity.

• Gametes: sex cells– Sperm: male gamete–Ovum: female gamete

• Fertilization: the process by which the male gamete unites with the female gamete.

• Zygote: The cell formed after the male and female gamete unite.

Monohybrid Cross

• A cross between two parents that differ only in one trait.

• Hybrid: The offspring of parents that have different forms of a trait.

Dominant and Recessive Traits

• Every trait is controlled by two factors.• Dominant trait: Trait that is expressed

whenever at least one factor for that form of the characteristic is present.

• Recessive trait: Trait that is not seen if a factor for the dominant form of the characteristic is present.

Alleles

• Gene: The “factors” described earlier. A trait that can be inherited.

• Alleles: Different forms of a gene.

Law of Dominance

• Each organism receives two alleles for a trait (one from each parent).

• The dominant trait appears if one or both alleles are for that trait.

• The recessive trait appears ONLY if both alleles are for that recessive trait.

Law of Segregation

• Gametes randomly receive one of the parent’s alleles for a trait.

Genotype

• The combination of alleles in an organism.– Homozygous: The two alleles for the trait are the

same.– Heterozygous: The two alleles for the trait are

different.

Phenotype

• The trait that the organism displays.

• Incomplete dominance: The offspring show a blend of their parents’ traits.

• Codominance: Both traits are displayed in the organism.

4.2 Punnett SquaresA.K.A “The Alien Lesson”

• Punnett square: A chart used to predict the possible genotypes of the offspring of a cross between two parent organisms.

• A capital letter is used to represent a DOMINANT allele, and a lower-case letter is used to represent a recessive allele.

Remember that homozygous means that the two alleles are the

same, and heterozygous means that the two alleles are different.

Our first example involves alien skin color.

• Blue skin color is dominant, and expressed with a capital B.

• Green skin color is recessive, and expressed with a lower-case b.

• The first parent is a homozygous, blue-skinned alien. Her genotype will be _______.

• The second parent is a homozygous, green-skinned alien. His genotype will be ______.

Our first example involves alien skin color.

• Blue skin color is dominant, and expressed with a capital B.

• Green skin color is recessive, and expressed with a lower-case b.

• The first parent is a homozygous, blue-skinned alien. Her genotype will be BB.

• The second parent is a homozygous, green-skinned alien. His genotype will be bb.

A Punnett Square for one trait is a box divided into four squares:

We write the genotype for one parent across the top, and the other along the left side (Law of Segregation).

It doesn’t matter which parent you write where. B B

b

b

Now we start the cross. In each box, write the letter from above and the letter from the side. It is

customary to write capital letter first in a heterozygote.

Bb

B B

b

b

Continue to fill in the other three boxes of the Punnett square.

Bb Bb

Bb Bb

B B

b

b

The genotypes (combinations of alleles) from this cross are all Bb. All of the offspring (4/4 = 1) can be expected

to have this genotype.

Bb Bb

Bb Bb

B B

b

b

The phenotypes (expressed traits) from this cross are all blue skin. All of the offspring (4/4 = 1) can be expected

to have this phenotype.

Bb Bb

Bb Bb

B B

b

b

Now, let’s cross two heterozygous blue-skinned aliens.

• The genotype for each of these aliens is _______.

Now, let’s cross two heterozygous blue-skinned aliens.

• The genotype for each of these aliens is Bb.

Now let’s write the alleles for each parent along the top and the side of the Punnett’s Square.

B b

B

b

And now we fill in the four boxes.

BB Bb

Bb bb

B b

B

b

The possible genotypes of this cross are BB (1/4), Bb (2/4 = 1/2), and bb (1/4).

BB Bb

Bb bb

B b

B

b

The possible phenotypes of this cross are blue skin (3/4) and green skin (1/4).

BB blue

Bb blue

Bb blue

bb green

B b

B

b

Alien Eye Number

• The three-eyes trait is dominant, represented with a capital E.

• The two-eyes trait is recessive, represented with a lowercase e.

• Genotype for homozygous two-eyed is _____.• Genotype for heterozygous three-eyed is

______.

Alien Eye Number

• The three-eyes trait is dominant, represented with a capital E.

• The two-eyes trait is recessive, represented with a lowercase e.

• Genotype for homozygous two-eyed is ee.• Genotype for heterozygous three-eyed is

Ee.

Now let’s write the alleles for each parent along the top and the side of the Punnett’s Square.

e e

E

e

And now fill in the boxes.

Ee Ee

ee ee

e e

E

e

The possible genotypes for this cross are Ee (2/4 = 1/2)and ee (2/4 = 1/2).

Ee Ee

ee ee

e e

E

e

The possible phenotypes for this cross are three-eyed (2/4 = 1/2) and two-eyed (2/4 = 1/2).

Ee three-eyed

Ee three-eyed

ee two-eyed

ee two-eyed

e e

E

e

Dihybrid cross

• A cross that looks at two different traits.• Generally, traits segregate independently, so

for two traits, there are four different combinations of alleles that the parent can pass on to its offspring.

• A heterozygous blue-skinned, heterozygous three-eyed alien’s genotype is BbEe.

• The possible combinations of alleles from this parent are:– BE– Be– bE– be

To do a dihybrid cross, we need a Punnett’s square divided into 16 boxes.

Each possible allele combination goes across the top and along the side.

BE Be bE be

BE

Be

bE

be

And now we fill in the boxes.

BBEE BBEe BbEE BbEe

BBEe BBee BbEe Bbee

BbEE BbEe bbEE bbEe

BbEe Bbee bbEe bbee

BE Be bE be

BE

Be

bE

be

Possible Genotypes:

BBEE 1/ 16 BBEe 2/16 = 1/8BBee 1/16BbEE 2/16 = 1/8BbEe 4/16 = 1/4Bbee 2/16 = 1/8bbEE 1/16bbEe 2/16 = 1/8bbee 1/16

Phenotypes:

BBEEBlue, 3

BBEeBlue, 3

BbEEBlue, 3

BbEeBlue, 3

BBEeBlue, 3

BBeeBlue, 2

BbEeBlue, 3

BbeeBlue, 2

BbEEBlue, 3

BbEeBlue, 3

bbEEGreen, 3

bbEeGreen, 3

BbEeBlue, 3

BbeeBlue, 2

bbEeGreen, 3

bbeeGreen, 2

BE Be bE be

BE

Be

bE

be

Possible phenotypes:

Blue skin and three eyes: 9/16 Blue skin and two eyes: 3/16Green skin and three eyes: 3/16Green skin and two eyes: 1/16

What is the phenotype for Dermed, our Intro to Health Care alien?

What is the phenotype for Dermed, our Intro to Health Care alien?

He is a green-skinned, three-eyed alien!

What are the possible genotypes for Dermed’s skin color?

What are the possible genotypes for Dermed’s skin color?

He has green skin, which is a recessive trait. Therefore, the only possible genotype is bb.

What are the possible genotypes for Dermed’s eye number?

He has three eyes, which is a dominant trait. Therefore, the possible genotypes are EE or Ee.

4.3 Chromosomes

• Chromosome: The structure that contains genes.

• Made of DNA (and some proteins to help it hold shape).

• Found in the nucleus of eukaryotic cells.

• Chromosomes look like thin threads until the cell is almost ready to divide.

• Just before cell division, the chromosome coils itself into an X-shape.

Sister chromatids

Chromatid: Each side of X-shaped chromosome.

Sister chromatids: The pair of chromatids that make up one chromosome. They are exact copies of each other (so that when the cell divides, each new cell gets a copy).

Centromere: The point at which the two sister chromatids are held together.

• In organisms that have two parents (that is, undergo sexual reproduction), the chromosomes occur in pairs (one from male parent, and one from female parent.)

• Humans have 23 PAIRS of chromosomes (46 total chromosomes).

• Homologous chromosomes: The two chromosomes that form each pair.

Human chromosomes

Mitosis

• Just before cell division occurs, the cells make a copy of each of their chromosomes and makes extra organelles and cytoplasm.

• Mitosis: The process by which the nucleus of a cell divides.

Mitosis has four stages:• Prophase: Chromosomes group together;

nuclear envelope disappears.• Metaphase: Chromosomes line up along center

of the cell.• Anaphase: Centromeres split; sister chromatids

move to opposite ends of the cell.• Telophase: Cell membrane pinches in at the

center of the cell; nuclear envelope reappears around the two groups of chromosomes.

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

• After mitosis is complete, the cell undergoes cytokinesis: the complete splitting of the cell into two identical daughter cells, each with a complete set of chromosomes.

Meiosis

• Remember that during sexual reproduction, a male gamete (sperm) unites with a female gamete (ovum) to create the zygote, which forms into the new organism.

Meiosis

• The new organism must have the same number of chromosomes as its parent organisms.

• If two regular body cells joined together to form the new organism, the new organism would have twice the number of chromosomes as the parents.

Meiosis

• Meiosis is the process by which gametes, which have HALF the number of chromosomes as the rest of the body’s cells, are formed.

Meiosis

• The process of meiosis is very similar to mitosis, but the cycle is repeated.

• During meiosis I, the cell divides into two cells, each with the original number of chromosomes.

• During meiosis II, the two cells formed in meiosis I divide again, creating a total of 4 cells, each with half of the number of chromosomes as the original cell.

Meiosis I

Meiosis II

Determining Gender

• Sex chromosomes: The pair of homologous chromosomes that determines the sex of the organism.

• Autosomes: The other pairs of homologous chromosomes, that determine all traits other than sex.

• Females have two X chromosomes (XX). They pass on only X chromosomes to their offspring.

• Males have one X and one Y chromosome (XY). They can pass on either an X chromosome or a Y chromosome to their offspring.– Therefore, the male determines the sex of his

offspring!

Sex-linked traits

• Some traits, other than sex related traits, are carried on the sex chromosomes (especially the X chromosome since it’s much bigger).

• Example: color blindness and hemophilia.• And this causes calico coloring in cats.

4.4 The Role of DNA

• DNA is made of smaller units called nucleotides.

• A nucleotide is made of a phosphate, a sugar, and a nitrogenous base.– Nitrogenous bases:• Adenine (A)• Thymine (T)• Guanine (G)• Cytosine (C)

DNA is shaped like a double helix (a twisted ladder).

• The sides of the ladder are made of alternating sugar and phosphate molecules.

• The rungs are made of bound nitrogenous bases.– Adenine and thymine always bind together.– Cytosine and guanine always bind together.

Alternating sugar and phosphate molecules

Adenine

Thymine

CytosineGuanine

DNA replication

• In order for the two daughter cells to receive full copies of the parent cell’s genetic information, the DNA must be copied (replicated).

The process of DNA replication

• The DNA strands separate (the bonds between the two bases that make up each rung separate) – like “unzipping.”

• New bases attach to each strand.• Two identical DNA molecules are formed

(each with one strand of old DNA and one strand of new DNA).

Mutation

• Any change in the DNA sequence.• Depending on the change, it may have little or

no affect, or it may be harmful.

Genetic code

• The order of the bases is the genetic code. The code determines how the proteins are made.

RNA

• RNA carries the information from the DNA in the nucleus to the ribosomes in the cytoplasm so that proteins can be made.

DNA and RNA are very similar, except:

DNA RNAStrands: Double-

strandedSingle-

strandedSugar: Deoxyribose RiboseBases: G. C, A, T G, C, A,

URACIL (U)

Protein Synthesis

• THE DNA SENTENCE LAB!