AP Biology Heredity Unit 5 notes

Meiosis & Sexual Reproduction

Genetics: the study of heredity and heredity variation

Heredity: the transmission of traits from one generation to the next, from parents to offspring.

Genes: segments of DNA that code for the basic unit of heredity. These are the instructions for making proteins that carry out various function in the body.

Inheritance: offspring acquire genes from their parents by inheriting chromosomes. Each gamete (sperm or egg carries a unique combination of genes, ensuring genetic diversity.

Cell division / Asexual reproduction

mitosis:

• produce cells with same information

  • identical daughter cells

  • exact copies (clones)

• same amount of DNA

  • same number of chromosomes

  • same genetic information

Asexual reproduction:

• single-celled eukaryotes

  • yeast (fungi)

  • protists

    • paramecium

    • amoeba

• simple multicellular eukaryotes

  • hydra

• disadvantages? no natural variation

Homologous Chromosomes:

• paired chromosomes (one from each parent) that are similar in size, shape, and genetic content. the carry the same genes, although the specific versions (alleles) of these genes may differ

  • both chromosomes of a pair carry “matching” genes

    • control same inherited characters

    • homologous = same information

  • genetic variation: during meiosis homologous chromosomes can exchange segments through the process called crossing over, leading to genetic recombination. This results in four unique gametes, each with different combination of alleles, increasing genetic diversity in offspring

how do we make sperm and eggs?

• must reduce 46 chromosomes → 23

  • must reduce the number of chromosomes by half

Meiosis: production of gametes

• alternating stages

  • chromosome number must be reduced

    • diploid → haploid

    • 2n → n

      • humans: 46 → 23

    • meiosis reduces chromosome number

    • makes gametes

  • fertilization restores chromosome number

    • haploid → diploid

    • n → 2n

Meiosis

• specialized type of cell division that results in four genetically unique haploid cells, crucial for sexual reproduction. It ensure genetic diversity and reduces the chromosome number by half, which is essential for forming gametes (sperm and eggs in animals, pollen and ovules in plants)

• reduction division

  • special cell division for sexual reproduction

  • reduce 2n → 1n

  • diploid → haploid

    • “two” → “half”

  • makes gametes

    • sperm, eggs

Preparing for meiosis

• 1st step of meiosis

  • duplication of DNA

  • why bother?

    • meiosis evolved after mitosis

    • convenient to use. “machinery” of mitosis

    • DNA replicated in S phase of interphase of meiosis (just like in mitosis)

Steps of meiosis

• meiosis 1

  • interphase (preparation: the cell undergoes G1, S / where DNA replication occurs, and G2 phases. chromosomes are in the chromatin form)

  • prophase 1 (synapse: homologous chromosomes pair up to form tetrads / four sister chromatids) (crossing over: occurs at the chiasmata, where homologous chromosomes exchange genetic material, increasing genetic diversity)

  • metaphase 1 (independent orientation: tetrads line up randomly align the metaphase plate. the arrangement is random, which contributes to genetic variation)

  • anaphase 1 (homologous chromosome separation: homologous chromosomes are pulled to opposite poles. sister chromatids remain attached.)

  • telophase 1 & cytokinesis 1 (formation of two haploid cells: each cell now has a haploid set of chromosomes, but each chromosome still consists of two sister chromatids)

    • 1st division of meiosis separates homologous pairs ( 2n → 1n ) “reduction division”

• meiosis 2

  • prophase 2 (spindle formation: the spindle apparatus forms in each haploid cell, and chromosomes condense)

  • metaphase 2 (chromosome alignment: chromosomes line up at the metaphase plate similar to mitosis, but with half the chromosome number)

  • anaphase 2 (sister chromatid separation: sister chromatids are separated and move towards opposite poles of the cell)

  • telophase 2 & cytokinesis (formation of four haploid cells: nuclei reappear around each set of chromosomes, and the cells divide, resulting in four genetically unique haploid cells)

    • 2nd division of meiosis separates sister chromatids (1n → 1n) “just like mitosis”

key takeaways:

• haploid cells: meiosis produces four haploid cells from a single diploid parent cell, each with half the number of chromosomes (e.g 23 in humans)

• genetic diversity:

  • crossing over: occurs in prophase 1, creating genetic diversity

  • independent assortment: during metaphase 1, chromosomes are distributed randomly, contributing to genetic diversity

  • random fertilization: the combination of gametes during fertilization adds to genetic variability.

• nondisjunction: if chromosomes or sister chromatids fail to separate properly during meiosis, it results in gametes with abnormal chromosome numbers, which lead to genetic disorders

Meiosis 1

• 1st division of meiosis

• separates homologous pairs

Trading pieces of DNA

• crossing over

  • during prophase 1, sister chromatids intertwine

    • homologous pairs swap pieces of chromosome

      • DNA breaks & re-attaches

Crossing over

• 3 steps

1. cross over

2. breakage of DNA

3. re-fusing of DNA

• new combination of traits

Meiosis 2

• 2nd division of meiosis

• separates sister chromatids

mitosis vs meiosis

The value of sexual reproduction

• sexual reproduction introduces genetic variation

  • genetic recombination

    • independent assortment of chromosomes

    • random alignment of homologous chromosomes in Metaphase 1

  • crossing over

    • mixing of alleles across homologous chromosomes

  • random fertilization

    • which sperm fertilizes which egg?

• dividing evolution

  • providing variation for natural selection

Variation for genetic recombination

• independent assortment for chromosomes

  • meiosis introduces genetic variation

  • gametes of offspring do not have same combination of genes as gametes from parents

    • random assortment in humans produces 8,388,608 different combination in gametes

Variation from crossing over

• crossing over creates completely new combination of traits on each chromosome

  • creates an infinite variety in gametes

Variation from random fertilization

• sperm + egg = ?

  • any 2 parents will produce a zygote with over 70 trillion possible diploid combination

Sexual reproduction creates variability

• sexual reproduction allows us to maintain both genetic similarity and differences

Sperm production

• spermatogenesis

• continuous & prolific process

• each ejaculation = 100-600 million sperm

Egg Production

• Oogenesis

  • eggs in ovaries halted before Anaphase 1

  • Meiosis 1 completed during maturation

  • Meiosis 2 completed after fertilization

  • 1 egg + 2 polar bodies

Differences across kingdoms

• not all organisms use haploid & diploid stages in same way

  • which on is dominant (2n or n) differs

  • but still alternate between haploid & diploid

    • must for sexual reproduction

Mendelian Genetics

Common Ancestry

• DNA and RNA carry genetic information

• The genetic code is shared by all living systems

• Gregor Mendel studied inheritance and created two laws that can be applied to the study of genetics

Gregor Mendel

• Mendel was an Austrian monk who experimented on pea plants and discovered the basic principles of heredity.

• why pea plants?

  • many varieties

  • controlled mating

  • relatively short generation time

Pea Plant Traits

• Mendel only tracked characteristics that came in two distinct forms:

• examples:

  • color (purple or white)

  • seed shape (round or wrinkled)

• to help control his experiments, he used true breeding plants

  • true breeding: organisms that produce offspring of the same variety over many generations of self-pollination.

    • example: true-breeding purple pea plants will only produce purple offspring with self-pollination.

Generations

• P generation: true-breeding parental generation

• F1 generation: (first filial) hybrid offspring of P generation (only displays one of the parents traits)

• F2 generation: (second Filial) offspring of the F generation (both parents traits displayed. 3:1 observed ratio dominant to recessive.

  

Punnett Squares

• diagrams used to predict the allele combinations of offspring from a cross with known genetic compositions

• capital letter denote dominant traits

• lower case letters denote recessive traits

Genetics Vocabulary

• Homozygous: an organism that has a pair of identical alleles for a character

  • example:

  • homozygous dominant: AA

  • homozygous recessive: aa

• Heterozygous: an organism has two different alleles for a gene

  • example:

  • Aa

• Genotype: the genetic makeup (alleles) for an organism

• Phenotype: an organisms appearance, which is determined by the genotype

Testcross

• helps to determine if the dominant trait is homozygous dominant or heterozygous

Principles of Heredity

• Mendel’s experiments allowed him to develop two fundamental principles of heredity:

  1. The law of segregation

  2. The law of independent assortment

Discoveries

• Mendel noticed that the cross between purple and white true breeding pea plants produced only purple F1 offspring

  • Did the white characteristic disappear?

    • No, because the white pea flower characteristic came back in the F2 generation

Dominant vs Recessive

• Mendel hypothesized that the purple must be a dominant trait to the white flower, which is a recessive trait

• Mendel performed the same crosses for each of the seven characteristics of pea plants and found the same results

  • he found that the F2 generation was always a 3:1 ratio

Mendel’s Model

• to explain the 3:1 ratio he observed in the F2 generation, Mendel created a model with four concepts:

1. Alternative versions of genes (alleles) account for variations in inherited characteristics

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

3. If two alleles at a locus differ, then the dominant allele determines the appearance and the recessive alleles has no noticeable effect

4. Law of segregation: the two alleles for the same trait separate during gamete formation and end up in different gametes

Alleles: A closer look

• Alleles: Different versions of the same gene.

• somatic cells are diploid

  • they contain two copies of each chromosome

  • Alleles: alternative versions of a gene

    

The Law of Segregation:

definition: during meiosis, only one allele for a trait is passed on to each gamete. this means that the two alleles for a gene separate so that each gamete receives only one allele.

implication: each parent contributes one allele for each trait to their offspring.

• During gamete formation, the two alleles for each gene segregate (separate) from each other.

• This means that each gamete receives only one allele for each gene.

• Key concepts:

• Alleles: Different versions of the same gene.

• Homologous Chromosomes: Pairs of chromosomes, one from each parent, that carry genes for the same traits.

• Meiosis: The process of cell division that produces gametes (sperm and egg cells).

• Significance:

  • Explains how genetic variation is maintained in populations.

  • Provides a foundation for understanding the inheritance of traits.

• Visual Representation: Often depicted using Punnett squares to predict the possible offspring genotypes and phenotypes from a cross between two individuals.

Monohybrid Crosses

• the law of segregation was determined by doing crosses between true-breeding plants which produced F1 hybrids, known as monohybrids

  • examples: BB x bb produce F1 that are all Bb

  • Monohybrid Crosses: a cross between the F1 hybrids

  • BB x Bb

The law of Independent Assortment

definition: alleles for separate traits are passed on independently of each other, provided the genes are not linked on the same chromosome.

implication: the inheritance of one trait generally does not affect the inheritance of another trait.

• Mendel’s second principle is the law of independent assortment: genes for one trait are not inherited with genes of another trait

  • instead of following one trait in his crosses, this time Mendel followed Two traits (i.e. pea pod color and pea pod shape)

• This law only applies to: genes that are located on different chromosomes (not homologous) OR genes that are very far apart on the same chromosome

Dihybrid Crosses

• the law of independent assortment was determined by doing crosses between plants that were true breeding for two traits, which produces F1 hybrids known as dihybrids

• example: YYRR x yyrr

  • all F1 dihydrides would be YyRr

• Dihybrid cross: a cross between F1 dihybrids

  • YyRr x YyRr

How to solve Genetics Problems

1. Write down the symbols for the alleles (sometimes they are given to you)

2. Write down the genotypes given

   a. if phenotypes are given, then write down the possible genotypes

3. determine what the problem is asking, and write out the cross as: [genotype] x [genotype]

4. set up the Punnett square

Law of Probability

• the law of segregation and independent assortment reflect rules of probability.

• The multiplication rule: the probability that two or more independent events will occur together in some specific combination

• example: if you flip a coin twice, what is the probability that it will land heads up both times?

  • ½ x ½ = ¼

• example: what is the probability of having 3 girls in a row?

  • ½ x ½ x ½ = 1/8

• The addition rule: the probability that two or more mutually exclusive events will occur

• example: what is the chance of rolling a dice and it lands on a 1 or 6?

  • 1/6 + 1/6 = 1/3

Pedigrees

• many human traits follow Mendelian patterns of genetics

• Pedigrees: family trees that give a visual of inheritance patterns of particular traits.

Reading Pedigrees

circles: represent females

squares: represent males

filled in symbols: indicate individuals who express the trait being studied

autosomal traits: equally likely to affect males and females

sex-linked traits: more likely to affect one sex over the other, often found on the X chromosomes. males (XY) are more likely to express X-linked recessive traits because they only have one X chromosome.

Sex linked recessive: rarely affect females (XX) because they only need two copies of the recessive allele to express the trait, whereas males need only one.

• if a trait is dominant, one parent must have the trait

  0 dominant traits do not skip a generation

• if a trait is X-linked, then males are more commonly affected than males.

Dominant: affected individuals appear in every generation

Recessive: trait skips generations

Non-Mendelian Genetics

many traits do not follow the ratios predicted by Mendel’s laws. why?

• varying degrees of dominance

• many traits are produced through multiple genes acting together

• some traits are determined by genes on the sex chromosomes

• some genes are adjacent or close to one another on the same chromosome and will segregate as a unit

• some traits are the result of non-nuclear inheritance (ie chloroplast and mitochondrial DNA)

Degrees of Dominance

Alleles can show varying degrees of dominance

• in Mendel’s experiments, he worked with traits that showed complete dominance

  • homozygous dominant and heterozygous individuals are phenotypically the same

• incomplete dominance: neither allele is fully dominant

  • F1 generation has a phenotype that is a mix of those of the parental generation

  • example: red flowers crossed with white flowers will produce pink offspring

• codominance: two alleles that affect phenotype are both expressed

  • example: human blood group

  • type AB blood: A and B are both expressed

• Multiple Alleles: genes that exist in forms with more than two alleles

  • example: human blood group

  • Alleles: I ^A , I ^B , i

• Pleiotropy: a single gene influences multiple phenotypic traits

  • example: in pea plants, the gene that controls flower color also affects seed coat color

Multiple Genes

in many cases, two or more genes are responsible in determining phenotypes.

Epistasis: the phenotypic expression of a gene at one locus affects a gene at another locus

• example: coat color in labs and some mice 

  • One gene codes for pigment and a second gene determines whether or not that pigment will be deposited in the hair

    

• Polygenic inheritance: the effect of two or more genes acting on a single phenotype

• example: height, human skin color 

  

Sex Chromosomes

Sex linked Genes: Thomas Hunt Morgan experimented with fruit flies and determined that specific genes can be carried on sex chromosomes

• sex linked gene: a gene located on either the X or the Y chromosome

• Y-linked genes: genes specifically found on the Y chromosome

  • very few Y linked genes, so very few disorders

• X-linked genes: genes found on the X chromosome

Inheritance of X-linked Genes:

• fathers can pass X-linked alleles to all of their daughters, but none to their sons

• mothers can pass X-linked alleles to both daughters and sons

  If an X-linked trait is due to a recessive allele:

  • females will only express trait if they are homozygous

  • because males only have one X chromosome, they will express the trait if they inherit it from their mother

    • they are called hemizygous (since the term heterozygous does not apply)

    • due to this, males are much more likely to have an X-linked disorder

  X - linked Disorders

• Duchenne muscular dystrophy

  • progressive weakening of muscles

• Hemophilia

  • inability to properly clot blood

• Color Blindness

  • inability to correctly see colors

X - inactivation:

• females inherit two X chromosomes, which is double than males!

• during the development, most of the X chromosome in each cell becomes inactive

  • the inactive X in each cell of a female condenses into a Barr body

    • helps to regulate gene dosage in females

Linked Genes

Genetic Recombination:

• Genetic recombination: production of offspring with a new combination of genes from parents

• Parental types: offspring with the parental phenotype

• Recombinants: offspring with phenotypes that are different from the parents

  Mendel also observed recombinants during his crosses

• example: green wrinkled plant crossed with a yellow-round plant

  • yyrr x YyRr

• 50% recombination, however, indicates that genes are unlinked, or on different chromosomes

• Linked genes: genes located near each other on the same chromosome that tend to be inherited together

• meiosis and random fertilization generate genetic variation in offspring due to:

  • independent assortment of chromosomes

  • crossing over in meiosis I

  • any sperm can fertilize any egg

• linked genes: crossing over

• linked genes show parental phenotypes in offspring at higher than 50%

• during crossing over chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments

• crossing over helps to explain why some linked genes become separated during meiosis

• the further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur between them and the higher the recombination frequency

• mapping distance:

  experiments performed by Sturtevant, allowed scientists to map genes and their locations on chromosomes

  linkage map: genetic map that is based on recombination frequencies

  • the distance between genes are map units

    • one map unit is equivalent to a 1% recombination frequency

• express the relative distances along chromosomes

• 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes

Non-Nuclear DNA

some traits are located on DNA found in the mitochondria or chloroplasts

• both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells

  • in animals, mitochondria are transmitted by the egg, NOT the sperm

  • in plants, mitochondria and chloroplasts are transmitted in the ovule, NOT the pollen

    • therefore, both mitochondria and chloroplasts determined traits are maternally inherited

Statistical Analysis: Chi Square

• Chi-square: a form of statistical analysis used to compare the actual results (observed) with the expected results

• help to:

  • determine whether the data obtained experimentally provides a “good fit” to the expected date

  • determine if any deviations from the expected results are due to random chance alone or to other circumstances (ie data collection error)

• designed to analyze categorical data

• Chi square (X ^ 2)

• use the equation to test the “null” hypothesis

  • the prediction that data from the experiment will match the expected results

• formula:

  

∑ = sum

how to solve chi square:

1. State Your Hypotheses

• Null Hypothesis (H₀): This usually states there is no significant difference between observed and expected data. Any differences are due to chance.

• Alternative Hypothesis (Ha): This states there is a significant difference between observed and expected data./

2. Set Up a Table

Create a table to organize your data:

3. Calculate Expected Values

• Equal Probability: If you expect an equal distribution among categories, divide the total number of observations by the number of categories.

• Based on Ratios: If you have a specific ratio (e.g., from a Punnett square), apply that ratio to the total number of observations.

4. Fill in the Table

• Observed (O): Record the actual counts from your experiment or data.

• Expected (E): Record the expected counts you calculated.

• (O - E): Subtract the expected value from the observed value for each category.

• (O - E)²: Square the difference you just calculated.

• (O - E)² / E: Divide the squared difference by the expected value for each category.

5. Calculate the Chi-Square Value (x²)

Sum the values in the last column of your table: x² = Σ [(O - E)² / E]

6. Determine Degrees of Freedom (df)

Subtract 1 from the number of categories: df = (number of categories) - 1

7. Find the Critical Value

• Use a chi-square distribution table. These are usually provided in AP Biology resources or exams.

• Locate the critical value that corresponds to your degrees of freedom and the significance level (usually 0.05 in AP Biology).

8. Compare Your Chi-Square Value to the Critical Value

• If your x² value is greater than the critical value: Reject the null hypothesis. The difference between observed and expected data is statistically significant.  

• If your x² value is less than or equal to the critical value: Fail to reject the null hypothesis. The difference between observed and expected data is likely due to chance.

9. Conclusion

• State whether you reject or fail to reject the null hypothesis.

• Explain what this means in the context of your experiment or data.

• If you reject the null hypothesis, you might suggest possible reasons for the significant difference.

Important Notes:

• The chi-square test is used to analyze categorical data (counts or frequencies).

• The expected values should generally be greater than 5 for each category to ensure the validity of the test.

• AP Biology questions often require you to not only calculate the chi-square value but also interpret the results in the context of the problem.

Environmental Effects on Phenotype

environmental factors:

• various environmental factors can influence gene expression and lead to phenotypic plasticity

  • individuals with the same genotype exhibit different phenotypes in different environments

  • examples:

  • temperature can change coat color in rabbits and Siamese cats

  • soil pH can affect flower color

  • UV exposure can increase melanin production in the skin

Chromosomal Inheritance and Disorders

genetic disorders

• some genetic disorders can be linked to affected or mutated alleles or chromosomal changes

mutated alleles

• Tay-sachs disease

  • autosomal recessive disease

    • mutated HEX gene

    • body fails to produce an enzyme that breaks down a particular lipid

    • affects central nervous system and results in blindness

• sickle cell anemia

  • autosomal recessive disease

  • mutated HBB gene

  • sickled cells contain abnormal hemoglobin molecules

chromosomal changes

• nondisjunction: chromosomes fail to separate properly in meiosis I or meiosis II

• karyotyping can detect nondisjunction

• example: down syndrome

  • three copies of chromosome 21