AP Bio Unit 5: Heredity

Somatic cells

“body” cells and contain normal number of chromosomes (diploid, 2n). skin cells, brain cells, etc.

Gametes

“sex” cells and contain half the normal number of chromosomes (haploid, n). sperm cells and ova (eggs).

Diploid (2n)

two sets of genetic information in homologous chromosome pairs

Haploid (n)

one set of genetic information in single copies of each chromosome

Homologous

pairs of chromosomes in a diploid organism that have the same structure, gene sequence, and loci but have different alleles

Meiosis

cell division to produce gametes (sperm or egg). 2n to n. necessary for sexual reproduction. two divisions.

Fertilization

the fusion of a sperm and egg to form a zygote (fertilized egg)

Zygote

fertilized egg

Homologous chromosomes carry…

genes controlling the same inherited traits

Locus

the location of the gene on a region of a chromosome

23 pairs

humans have _____ of homologous chromosomes

Humans have…

22 pairs of autosomes and 1 pair of sex chromosomes

Karyotype

method of organizing the chromosomes of a cell in relation to number, size and type

Autosomes

code for most of the offspring’s traits. sets 1-22.

Sex chromosomes

determine sex. 23rd set.

Interphase 1 (meiosis 1)

Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

Meiosis 1 phases

Prophase 1, metaphase 1, anaphase 1, telophase 1

Prophase 1

longest and most complex phase (90%). chromosomes condense. synapsis occurs.

Synapsis (prophase 1)

homologous chromosomes come together to form a tetrad (2 chromosomes/ 4 chromatids)

Tetrad

two pairs of homologous chromosomes next to each other

Chromatid

one of the two identical halves of a chromosome that has been replicated in preparation for cell division.

Non-sister chromatids

couples having the same length, patterns, and position of the centromere

Sister chromatids

two exactly similar copies of a chromatid. have the same genes.

Centromere

links a pair of sister chromatids together during cell division

Crossing over

occurs after synapsis (prophase 1). segments of non-sister chromatids are exchanged. increases genetic variation.

Crossing over forms…

new recombinant chromosomes

Crossing over location

occurs at a site called the chiasma (plural: chiasmata)

Recombination

a process by which pieces of DNA are broken and recombined to produce new combinations of alleles

Metaphase 1

shortest phase. tetrads align on the ___ plate. independent assortment occurs. orientation of homologous pairs is random.

Independent assortment

alleles of two genes will segregate into daughter cells independent of one another

2ⁿ

number of possible configurations in metaphase 1

Anaphase 1

homologous chromosomes separate and move towards the poles. sister chromatids remain attached at their centromeres.

Telophase 1

each pole now has a haploid set of chromosomes. cytokinesis follows and two haploid daughter cells are formed.

Cytokinesis

the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells

Meiosis 2

no interphase 2 (no dna replication). almost identical to mitosis.

Prophase 2

spindle fibers attach to the centromeres of the chromosomes

Metaphase 2

chromosomes line up along the metaphase plate in the center of the plate

Anaphase 2

sister chromatids separate

Telophase 2

2 nuclei form. cytokinesis follows. 4 haploid daughter cells produced. all genetically different.

Germ cell

can either go to mitosis to produce other germ cells or it can undergo meiosis in order to produce gametes

Synaptonemal complex

protein structure that holds homologues together

Chiasmata

cross-shaped structures where homologues are linked together. keep the homologues together after the synaptonemal complex breaks down.

Oogenesis (egg production)

only one functional egg is made while non-eggs (polar bodies) undergo apoptosis (programmed cell death)

Gene linkage

genes inherited together or in recurring groups. genes don’t assort independently.

Crossover events

genes that are father apart are more likely to be separated by _____

Gene mapping

the process of determining the location of genes on chromosomes. rate of linkage allowed scientists to do this.

Linked genes examples

Red hair/fair skin, red hair/freckles, brown hair/curly hair, brown hair/brown eyes, blonde hair/ blue eyes, blonde hair/fair skin

Genes

segements of DNA

Alleles

different forms of a gene

Dominant allele

masks a recessive

Recessive allele

expressed when dominant is not present

Genotype

what allele combination an organism has

Phenotype

physical manifestations of genes

The law of dominance (mendel’s law 1)

there are dominant and recessive alleles

The law of segregation (Mendel’s law 2)

During meiosis 1, the two alleles that an organism carries are randomly split. each gamete recieves just one copy of the gene. 50% of gametes have one allele and 50% have the other.

Sex linked (X-linked) traits

Males will always express the one gene they have. only females may be carriers of a recessive trait. males more likely to be affected by recessive traits (ex: baldness, colorblindness)

Incomplete dominance

A third (new) phenotype for heterozygous. Intermediate blend between dominant and recessive phenotypes.

Codominance

both traits expressed equally

Dihybrid crosses

two traits crossed at once

The law of independent assortment (mendel’s law 3)

any combination of alleles is possible in a gamete

If A and B are independent then

P(A and B) = P(A) * P(B) (multiply)

A and B are mutually exclusive then:

P (A and B) = P(A) + P(B)

Epistasis

The expression of one gene is affected by another. Genes can mask or later each other. Can result in new phenotypes.

Recombination frequency

recombinations have different combinations of genes than either parent

Chi-square analysis

used to determine if the differences between two data sets are statistically significant or due to chance alone

Chi square equation

χ² = ∑(O_i – E_i)²/E_i

Chi square analysis process

1. Formulate null hypothesis

2. Find your critical value (p-value almost always .05)

3. Compare critical value to chi square statistic

4. Greater - reject null, less than - accept null

Critical value

the intersection of your degrees of freedom and alpha (cut-off p value)

Phenotypic plasticity

phenotype is flexible and can be influenced by the environment (ex: temperature). One genotype can yield multiple phenotypes.

Nondisjunction

Homologous chromosomes do not separate properly during meiosis (anaphase 1 or 2). Results in abnormal number of chromosomes in gametes

Down syndrome

Trisomy 21

Turner syndrome

single X

Kinefelter syndrome

XXY

Pedigree charts

show a family record of a hereditary condition. used to study transmission of disease, physical traits, etc. generations identified by Roman numerals. Individuals identified by Arabic numerals from the left.

Autosomal recessive conditions

the parents of the affected individuals are not affected themselves. it skips a generation and few individuals are affected. (ex: cystic fibrosis, sickle cell anemia, albinism)

Autosomal dominant conditions

two affected parents have an unaffected child. every affected person has an affected parent and many are affected. (ex: cleft chin, huntington’s disease, marfan syndrome)

X-Linked recessive

More common in males. Females only express if homozygous recessive. Fathers never pass traits to sons. Father’s X is only passed to daughters. Only females can be carriers. Sometimes shown as half-shaded. Usually skips a generation. (ex: color blindness, baldness, haemophilia)

X-linked dominant

Affected fathers pass to all daughters but never sons. Both males and females may inherit from their mother. (100% if XA XA, 50% if XA Xa). Equal proportions of males/females. NO CARRIERS. (ex: vitamin D-resistant Rickets)

Pleiotropy

Some genes affect many different characteristics, not just a single one

Lethal alleles

some genes have alleles that prevent survival when homozygous or heterozygous

Complementary genes

recessive alleles of two different genes may give the same phenotype

Incomplete penetrance

not all individuals with a genotype display a corresponding phenotype

Variable expressivity

individuals of particular genotype may have stronger or weaker versions of a phenotype

Hemizygous

having only one copy of the gene

Mitochondria and chloroplasts

have their own DNA

Extranuclear inheritance

transmission of genes that occur outside of the nucleus

Mitochondrial inheritance

mitochondria and mtDNA are inherited maternally, as mothers always pass on their mitochondria and their mtDNA to their children. fathers never pass mtDNA.

Continuous variation

a range of phenotypes (ex: height)