Heredity

Chapters 13, 14, and 15

Meiosis and Sexual Life

CyclesChapter 12

An introduction to Heredity Offspring acquire genes from parents by

inheriting chromosomes. What are genes? Segments of DNA that each code for specific

traits. “Like begets like”, more or less

Sexual vs. Asexual Reproduction? Asexual reproduction produces a “clone” Sexual reproduction results in greater variation. why?

Sexual Life Cycles: Alternation of Fertilization and MeiosisHuman Life Cycle:

Somatic cells have 46 chromosomes Chromosomes differ by their size, positions of

the centromeres, and banding patterns. What are karyotypes? A display that pairs homologous chromosomes

of an individual Sex chromosomes vs. autosomes

Gametes – sex cells that have 22 autosomes and 1 sex chromosome

Haploid cells vs. Diploid cells Fertilization (syngamy) is the union of two

gamates resulting in a fertilized egg (zygote) Mitosis vs Meiosis Mitosis produces somatic cells Meiosis produces gametes

What does n and 2nrepresent?

Similarities andDifferences to Mitosis?

Meiosis Unlike mitosis, it divides twice (meiosis I and

meiosis II) Meiosis I – homologous chromosomes

separate Meiosis II – sister chromatids separate

(analogous to mitosis)

Prophase I Begins like prophase of mitosis:

1. Nucleus disappears

2. Chromatin condense into chromosomes Key difference is that Homologous

chromosomes pair (synapsis) These pairs are called tetrads (group of 4

chromatids) Crossing Over occurs at sites called

chiasmata (referred to as synaptonemal complex)

Metaphase I

1. Homologous pairs of chromosomes spread across metaphase plate

2. Microtubules are attached to kinetochores on one member of each pair

Anaphase I

1. Homologous tetrads uncouple and are pulled to opposite poles of cell

Telophase I

1. Chromosomes reached opposite poles

2. Each pole will form new nucleus with half the number of chromosomes from parent cell, but each chromosomes contain two chromatids.

At this point many species begin cytokinesis while others delay it until after meiosis II. A short interphase II may begin, but no replication of chromosomes occurs.

Prophase II

1. Nuclear Envelope disappears

2. Spindle develops

3. No chiasmata or crossing over (as in prophase I)

Metaphase II

1. Chromosomes align singly on the metaphase plate (no tetrads)

Just like mitosis, except now only half the number of chromososomes.

Anaphase II1. Chromosomes pulled apart into chromatids

2. Chromatids (now chromosomes) migrate to opposite poles

Like mitosis, except with only half the chromososomes

Telophase II

1. Nuclear Envelope reappears and cytokinesis occurs

2. End result = 4 haploid cells (1/2 the number of chromosomes and consists of only one chromatid)

Later (in Interphase) a 2nd chromatid is replicated for each, but cell will still only have half the number of chromososomes.

Summary (f 13.7)Event Mitosis Meiosis

DNA Replication

Number of

Divisions

Synapsis of homologous chromosomes

Number of daughter cells and genetic composition

Role in the animal body

Genetic Variation

In meiosis there is a reassortment of genetic material called genetic recombination which originates from three events:

1. Crossing Over

2. Independent Assortment of homologues

3. Random joining of gametes

Crossing Over

During Prophase I Nonsister chromatids of homologous

chromosomes exchange pieces of genetic material (what is the result?)

Each homologue would no longer entirely represent a single parent

Independent Assortment of Homologues:1. During metaphase I

2. Tetrads of homologous chromosomes separate into chromosomes that go to opposite poles.

3. Which chromosomes goes where depends on orientation and therefore seperation is random for each tetrad.

Maternal and Paternal chromosomes are inner mixed at each pole

Random Joining of Gametes1. Which sperm fertilizes which egg is to a

large degree a random event

2. Can be affected by genetic composition of the gamete (i.e. some sperm may be faster swimmers)

Why Do Cells Divide? (revisited) Relative Size to surface area ratio:

When there is a large surface area relative to volume ratio, then the cell can efficiently react with the outside environment (i.e. adequate amounts of O2 can diffuse in while waste products can rapidly be eliminated)

Limited Capability of the nucleus: Genome “controls” cell by producing needed

substances which regulate activities. The capacity of the genome is limited by its finite amount of genetic material

Mendel and the Gene Idea

Chapter 14

Probability Review: The rule of multiplication: to determine the

probability of two or more independent events occurring together, you merely multiply the probabilities of each event happening separately

Ex: For 2 consecutive coin tosses the chance to get heads each time is ½. So for getting two heads is ½ x ½ = ¼. Getting three heads is ½ x ½ x ½ = 1/8

Terminology Review: Gene Allele Locus Homologous pair Dominant Recessive Homozygous Heterozygous

Genotype Phenotype P, F1 and F2

generations Monohybrid Cross Dihybrid Cross Hybrids Testcross

Mendel’s Discoveries

Experimental and quantitative approach to genetics brought about Mendel’s discovery (what did he use?)

Law of Segregation Law of Independent Assortment Dominance and Recessive

Mendel’s Hypothesis:1. Alternative versions of genes (different alleles)

account for variations in inherited characters.

2. For each character, an organism inherits two alleles (one from each parent).

3. If the two alleles differ, then one, the dominant allele if fully expressed in the organism’s appearance; the other, the recessive allele, as no noticeable effct on the organism’s appearance.

4. The two alleles for each character segregate during gamete production

Law of Segregation Two Alleles for a character are packaged into

separate gametes. Each parent will contribute one allele for each

trait to the zygote Random segregation occurs during gamete

formation Determined by performing Monohybrid crosses Example: a heterozygous pea plant for height

would make one gamete with the trait for being tall while the other would have the trait for being short.

Law of Independent Assortment Each pair of alleles segregates into gametes independently

Factors for different characteristics are distributed to reproductive cells independently.

Determined by performing Dihybrid crosses Example: seed shape and seed color are

inherited independently of each other.

Punnet Square Practice: Round Seeds are dominant over wrinkled

seeds. Cross: Pure round seeds with Pure wrinkled seeds.

Green pods are dominant over yellow pods. Cross: Hybrid green pods with Hybrid Green pods.

Axial flowers are dominant over terminal flowers. Cross: Hybrid axial flowers with Pure axial flowers.

Colored seed coats are dominant over white seed coats. Cross: Hybrid colored seeds with Hybrid colored seeds.

Horned cattle is dominant over the hornless condition. Cross: Pure hornless with Hybrid Horned.

Black fur is dominant over white fur in guinea pigs. Cross: Hybrid black with Hybrid Black.

Long wings are dominant over curly wing in fruit flies. Show all the different crosses that can produce hybrid long wing individuals.

In Sheep, black wool is recessive to white wool. What happens when you mate a black ram to a heterozygous ewe? Use W to represent dominant white, w for the recessive black allele. What is the genotype of the ram? What is the genotype of the ewe? What are the genotypes of the offspring? What is the genotypic ratio of the offspring? What are the phenotypes of the offspring?

Cross a heterozygous black female angus to a heterozygous bull (B = black; b = red) What is the genotype of the female angus? What is the genotype of the bull? What are the genotypes of the offspring? What is the genotypic ratio of the offspring? What are the phenotypes of the offspring?

In cattle, the polled gene (P) is dominant over the horned gene (p). A polled cow with genotype (Pp) is mated to a horned bull. ½ of the offspring were polled and ½ were horned? What is the genotype of the bull? Whare the genotypes of the offspring?

Cross a heterozygous polled black angus bull (BbPp) to a heterozygous polled black angus cow (BbPp).

Use a punnet square to determine genotype and phenotype of offspring.

If plant heterozygous for all three characters “self-fertilizes”, what proportion of the offspring would

be expected to be as follows? (Use probability rules instead of a huge punnet square)

1. Homozygous for the three dominant traits?2. Homozygous for the three recessive traits?3. Heterozygous for the three recessive traits?4. Homozygous for axial and tall, heterozygous for seed

shape?

Character Dominant Recessive

Flower Position Axial (A) Terminal (a)

Stem Length Tall (T) Short (t)

Seed Shape Round (R) Wrinkled (r)

What is the probability that each of the following pairs of parents will produce the indicated offspring? (assume independent assortment of all gene pairs)

1. AABBCC x aabbcc AaBbCc

2. AABbCc x AaBbCc AAbbCC

3. AaBbCc x AaBbCc AaBbCc

4. aaBbCC x AABbcc AaBbCc

Extending Mendelian Genetics: “it was brilliant (or lucky) that Mendel chose

pea plant characters because they turned out to have relatively simple genetic basis: each character studied was determined by one gene, for which there are only two alleles, one completely dominant to the other

The relationship between genotype and phenotype is rarely so simple…

Incomplete Dominance True dominant and recessive behaviors are not

exhibited The heterozygous condition produces a blending

of the individual expressions of the two alleles. EX: Snapdragons: R = red flowers; r = white

flowers and Rr = Pink flowersSometimes both alleles are written with the same

upper-case or lower-case letter but with a prime or a superscript number (R and R’ )

H = Straight Hair; H1 = Curly Hair;

HH1 = intermediate phenotype expressed

Co-Dominance

A type of incomplete dominance Both inherited alleles are completely

expressed EX: M and N blood types produce molecules

that appear on the surface of human red blood cells. M produces 1 type of blood, while N produces another type. Those with MN produce both types of molecules

Three important points of Dominance/recessivness relationships:1. Range from complete dominance, through

various degrees of incomplete dominance, to co-dominance.

2. Reflect mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA.

3. Do not determine the relative abundance of alleles in a population.

Multiple Alleles Genes that exist in

populations in more than two forms

EX: A, B, O blood types This refers to a type of

carbohydrate found on the surface of red blood cells; type O is the absence of the carbohydrate.

How does this connect with things we’ve learned earlier?

Phenotype Genotypes

O ii

A IAIA or Iai

B IBIB or Ibi

AB IAIB

Connecting to blood transfusions What would happen if an individual with type B or O blood was given type A blood?

Their immune system would identify the wrong carbohydrate as a foreign substance (antigens) and produce antibodies to attack which results in agglutination (clumping of the blood cells which could lead to death).

AB is the universal recipient O is the universal donor

Epitstasis One gene affects the phenotypic expression of a

second gene Frequently occurs in expression of pigmentation One gene turns on (or off) the production of

pigment while a second controls either the amount of pigment produced or the color of pigment.

If the first gene turns it off, then the second gene has no effect, regardless of what it codes for.

EX: Mouse fur color (f14.11)

Pleiotropy Single gene has more than one phenotypic

expression EX: Gene in pea plants for round (R) or wrinkled

(r) seeds also influences the phenotypic expressions of starch metabolism and water absorption. Rounds seeds code for greater conversion of glucose to starch, while wrinkled seeds have more uncoverted glucose. Higher concentration of glucose then effects osmotic gradient…

Many disease-causing genes exhibit pleiotropy (sickle-cell anemia

Polygenic Inheritance Traits expressed in a continuous range of

varieties (not just two or three types)

Ex: Human Height – ranges from very short to very tall

Polygenic inheritance (many genes shaping one phenotype) is the opposite of pleiotropy (one gene influencing many phenotypes)

Pedigree Analysis

Reveals Mendelian patterns of human inheritance

Family pedigrees can be used to deduce possible genotypes of individuals and make predictions of future offspring (probabilities, not certainties)

Can be particular helpful in determining if genetic disorders are running in a family.

The Chromosomal Basis of Inheritance

Chapter 15

Thomas Morgan’s contribution: First to trace gene to a specific chromosome

Morgan studied Drosophilia melanogaster (fruit flies) Prolific Breeders Quick Life Cycles Only contains 4 pairs of chromosomes

Morgan’s studied provided chromosomal basis for Mendel’s laws.

Morgan crossed Wild-type flies (Red eyes) with “mutant phenotype” (White eyes)

His studies helped him to discover Sex-Linked Genes

Linked Genes Genes that reside on the same chromosome Why would they be considered linked? Cannot segregate independently because they

are physically linked How would this effect gamete formation? BbVv would only be able to make BV and bv

gametes thus making the expected genetic outcome of BV, bV, Bv, and bv possible.

Is there anyway to separate their link? Crossing Over

The greater the distance between two genes on a chromosome, the more places

between genes that the chromosome can break and crossing over will occur

Sex-Linked Inheritance Sex-linked genes are genes that reside on the X

chromosome (typically) or the Y chromosome. For females, inherited sex-linked genes is

similar to autosomal inheritance However, recessive sex-linked characteristics

are more common in males. Why is that? Males only inherit one copy of the gene

Examples: Hemophilia (cannot properly form blood clots); Color Blindness

X-Inactivation Occurs in female mammals During embryonic development, one X

chromosome uncoils while the other does not. The X chromosome remained coiled creates a

Barr Body Barr bodies are mostly inactive X chromosomes;

however, some remain active The selection of which X will become the Barr

body is random and independent in each of the embryonic cells; some will have active X from dad and others active X from mom (f15.10)

Ex: Calico cats

Nondisjunction Chromosomes do not properly separate during

Meiosis I Both members of homologous chromosome pair

migrate to the same pole

What is the result? Half of the gametes have an extra chromosome

and half will be missing a chromosome These gametes are usually sterile. Some imbalances are fertile, but almost always

lead to genetic defects

Examples of Genetic Defects caused by Nondisjunction: Down Syndrome (Trisomy 21) Turner Syndrome (XXY or XO)

Human Genetic Defects Genetic Defects can be caused by…1. Inheritance of an allele (hemophilia)2. Chromosomal abnormalities when inherited

genome is missing a chromosome or has an extra chromosome (result of nondisjunction)

3. One or more chromosomes have portions deleted (deletion), duplicated (duplication), moved to another chromosome (translocation), or rearranged in reverse orientation on the same chromosome (inversion)

AP expectations of students… Expects you to know the more common

genetic defects in humans and their underlying causes (been discussed throughout notes)

TABLE 6.1 (handout)

Genomic Imprinting Same alleles may have different effects on offspring

depending on whether they arrive in the zygote via the ovum or via the sperm.

In new generation, both maternal and paternal imprints are apparently “erased” in gamete-producing cells, and all the chromosomes are re-imprinted according to the sex of the individual in which they now reside.

Prader-Willi/Angelman case Fragile X syndrome (mental retardation) More common

when inherited from the motehr rather than the father.

Extranuclear genes Exhibit non-Mendelian pattern of inheritance Mitochondria and chloroplasts contain some of

their own genes. Since zygote’s cytoplasm comes from the ovum,

certain features of the offspring’s phenotype depend solely on these maternal cytoplasmic genes.

Some diseases affecting the nervous and muscular systems are caused by defects in mitochondrial DNA that prevent cells from making enough ATP