Unit 5 - Heredity

Diploid

two sets of chromosomes

• zygotic chromosome # = 2n

Haploid

one set of chromosomes

• zygotic chromosome # = n

Homologous chromosomes

duplicate versions of each chromosome

• similar in size & shape, contain same genes in same locations (although maybe diff. versions)

What are haploid cells in the human body?

sex cells or gametes

• each parent contributes a gamete to produce a zygote/diploid in offspring

Locus

position of a gene on a chromosome

Alleles

alternate versions of the same gene

Phenotype

physical traits

genotype

genetic makeup

Each generation in the cross

1) 1st generation = P generation (parent)

2) 2nd generation = F1 generation (filial)

3) 3rd generation = F2 generation

Law of Dominance

dominant allele masks the presence of the recessive allele

Law of Segregation

during gamete formation, the two alleles for each gene separate

• each sex cell only receives one allele → genetic variation

Law of Independent Assortment

during meiosis I, each pair of homologous chromosomes is split, and which one aligns left or right during metaphase is diff. for each pair

• two alleles for each trait are sorted independently of the two alleles for the other trait

Product Rule

if A and B are INDEPENDENT - events will occur simultaneously

• P(A + B) = P(A) x P(B)

Sum Rule

if A and B are MUTUALLY EXCLUSIVE - either event will occur

• P(A or B) = P(A) + P(B)

Linked genes

genes on the same chromosome that stay together during assortment and move as a group

• tends to be inherited together

• instead of 4 possible gamete combinations, there are only 2

Recombinants

offspring w/ new combinations of alleles that differ from their parents, created by genetic recombination during meiosis (crossing-over)

• causes linked genes to not be linked

• percentage of recombination: recombinants/total offspring, can also determine how far apart genes are in centimorgans

Frequency of crossing over

farther apart two linked alleles are, the more often the chromosome will cross over between them

Sex-linked traits

pair of chromosomes that determine sex, some traits are carried on sex chromosomes

• female = XX

• male = XY

Barr bodies

X chromosome that is condensed and visible

• in females, one X is activated and the other is deactivated during embryonic development

Incomplete dominance

in some cases, traits will blend - either is dominant over the other

• WW cross w/ RR = RW

Codominance

equal expression of both alleles - expression of one allele doesn’t prevent the expression of the other

• AB blood type

Polygenic inheritance

trait that results from the interaction of many genes, each gene will have a small effect on a particular trait

Non-nuclear inheritance

mitochondrial inheritance - maternal line

• mitochondria are always provided by the egg during sexual reproduction

• in plants, mitochondria are provided by the ovule & are maternally inherited

Phenotypic plasticity

two individuals w/ the same genotype that have diff. phenotypes because they are in different environments

Meiosis

production of gametes

• limited to sex cells in special sex organs called gonads

• male gonads = testes

• female gonads = ovaries

• female gamete + male gamete = zygote

Meiosis phases

Meiosis I

• prophase I, metaphase I, anaphase I, telophase I, cytokinesis I - ensures each gamete receives a haploid
  

Meiosis II

• prophase II, metaphase II, anaphase II, telophase II, cytokinesis II - results in four unique haploid daughter cells

Prophase I

• nuclear membrane disappears, chromosomes become visible, centrioles move to opposite poles of nucleus

• chromosomes line up side-by-side w/ homologs (synapsis)

• synapsis is followed by crossing-over - mixes alleles from dad & mom

Metaphase I

• chromosome pairs (tetrads) line up at the metaphase plate, in random order

Anaphase I

• each pair of chromatids within a tetrad moves to opposite poles

• homologs separate w/ their centromeres intact

Telophase I

• nuclear membrane forms around each set of chromosomes

Cytokinesis I

• two daughter cells are formed (haploid)

Prophase II

• chromosomes condense and become visible

Metaphase II

• chromosomes move toward metaphase plate, in single file not as pairs

Anaphase II

chromatids of each chromosome split at the centromere, and each chromatid is pulled to opposite ends of each cell

Telophase II

• nuclear membrane forms around each set of chromosomes

Cytokinesis II

• four haploid cells are produced

spermatogenesis

4 sperm cells are produced for each diploid cell

oogenesis

egg cell or ovum is produced (only 1), 3 polar bodies are produced

Nondisjunction

chromosomes that fail to separate during meiosis

• produces wrong # of chromosomes in a cell

• occurs during anaphase I or II

Down syndrome

3 copies of 21st chromosomes

• intellectual disability, developmental delays

• low muscle tone, small hands/feet

Turners syndrome

in females where one X chromosome is missing/altered, affecting growth, development, and fertility

• infertile

• short stature

• webbed necks/widely spaced nipples

autosomal vs sex-linked

• autosomal: genes on non-sex chromosomes (autosomes), affecting males and females equally

• sex-linked: genes on sex chromosomes, causing different patterns, often affecting males more for X-linked traits

chi-square test - when to reject/accept null hypothesis

reject: chi-square value > critical value - statistically significant difference in relationship, unlikely due to chance
accept: chi-square value < critical value - no significant difference, randomness

sexual vs asexual reproduction

• sexual: involves two parents, creates genetically unique offspring through fusion of gametes, more variation but requires more energy (animals)

• asexual: one parent, produces genetically identical clones via binary fission/budding, lacks variation but less energy (bacteria, plants)