BIO SCI 97: Lecture 8

recessive

Unaffected parents having affected children is a sign of autosomal _ inheritance

2/3

What is the probability that the circled individual is a carrier for cystic fibrosis?

1/4

If two heterozygous parents (Aa × Aa) have six children, and you know their youngest child is affected by the autosomal recessive disease, what is the probability that their next child will also be affected?

conditional probability

the probability of something changes because we know additional information

complement

a genetic _ refers to the complete set of genetic material that is present in a cell during the process of cell division

somatic

Mitosis occurs in _ cells, which encompass most of the cells throughout the body

mitosis

_ is involved in growth, repair, maintenance, and asexual reproduction of cells

G1

During the _ phase, the cell grows and synthesizes cytoplasmic materials for cell function and in preparation of division (proteins, organelles, other cellular components)

S

Each individual chromosome replicates in this phase, so you have 2 copies of each chromosome

46, 1, 46, 2, 92

Before S phase, there are _ chromosomes, each with _ chromatid(s)

After S phase, there are _ chromosomes, each with _ sister chromatid(s), so _ chromatids total

DNA, chromosomes

_ doubles, but the # of _ do not during S phase

G2

During _ phase, the cell checks that DNA replication has been completed and prepares the machinery needed for chromosome separation.

mitosis

phase of the cell cycle where:

• Chromosomes condense.

• Sister chromatids of replicated chromosomes separate and move to opposite poles of the cell.

• Two genetically identical daughter nuclei form, each containing the same chromosome number and genetic information as the parental nucleus before S phase.

germline cells

precursor cells that undergo meiosis to produce haploid reproductive cells, including specialized, diploid cells in the reproductive organs (testes, ovaries)

gametes

the mature reproductive cells used in sexual reproduction, which are haploid, and consist of sperm and egg cells

mitosis, meiosis

In _, the goal is to maintain the genetic complement

In _, the goal is to reduce the chromosome number by half in each of the 4 gametes (from parental → each gamete)

Independent assortment

the term to describe when in metaphase I of meiosis, each homologous chromosome pair lines up randomly and independently of the other pairs (each chromosome in a pair can be oriented towards either pole.) This produces different possible combinations of maternal and paternal chromosomes in gametes.

Crossing over

term for when Homologous chromosomes exchange corresponding DNA segments, creating new allele combinations.

genetic variation

Meiosis is a major source of _ _ , due to independent assortment and crossing over

somatic, germline

Mitosis occurs in _ cells, and meiosis occurs in _ cells

no

do homologous chromosomes always have two sister chromatids?

46, 46, 46, 92, 46, 46

Cell-cycle stage	Chromosomes	Chromatids
G1 (before replication)	a	b
G2 (after replication)	c	d
After mitosis, per daughter cell	e	f

Telomeres

protective DNA-protein structures at the ends of linear chromosomes, which help protect chromosome ends from damage and inappropriate joining

centromeres

specialized regions of chromosomes where kinetochores assemble, and spindle microtubules attach to these kinetochores to move chromosomes during cell division

microtubules

the protein structures that make up spindles, helping in chromosome movement

motor proteins

help to transport chromosomes to opposite poles of the cell, help in assembling and maintaining the mitotic spindle, which is vital for chromosome alignment and separation, and involved in the contraction of the contractile ring during cytokinesis, leading to the physical separation of the daughter cells.

1, 1, 1, 2, 2, 4, 1, 2

	Before S phase	After S phase
Chromosomes	a	b
DNA molecules	c	d
DNA strands	e	f
Chromatids	g	h

DNA

One chromosome consists of one long, continuous _ molecule

sister chromatid

During S phase, one double-stranded DNA molecule is replicated to produce two double-stranded DNA molecules, each of which forms the DNA of one _ _

No

Is a chromatid from the paternal homolog the sister of a chromatid from the maternal homolog?

identical

Sister chromatids are genetically _ because they are copies of the same original chromosome

chromatids, chromosomes

During the S phase, the number of _ doubles but the number of _ does not

cohesin

What protein complex holds sister chromatids together along their arms and at their centromeres after DNA replication and is cleaved during chromosome separation?

chromosomes

In anaphase of mitosis, each chromatid moving to opposite sides of the cell are each considered individual _

homologous

Chromosomes are _ because they carry the same genes at corresponding locations (same relative positions along chromosome)

type

One complete chromosome set contains one chromosome of every chromosome _.

Prophase I

During which phase of meiosis are 2 replicated homologous chromosomes paired into tetrads, allowing crossing over to occur?

one complete

In “n= x”, what does n represent?

The number of chromosomes in _ _ set

chromosomes

In “n= x”, what does x represent?

The total _ in that cell

chromatids, chromosomes

DNA replication doubles the number of chromosomes/chromatids, not the number of chromosomes/chromatids.

reductional, equational

Meiosis I is reductional/equational division, because the number of chromosomes is reduced by half from the starting cell to daughter cells, and meiosis II is reductional/equational, meaning that the number of chromosomes remains the same, but the number of chromatids reduce

I, II

In Meiosis _, the homologous chromosomes separate

In Meiosis _, the sister chromatids separate

separase

What enzyme does the cleaving of cohesin (protein complex) during the separation of sister chromatids?

Prophase I

During _ _ of meiosis, homologous chromosomes physically pair with each other

DNA sequences

Different alleles occur when the same gene at corresponding locations (loci) on homologous chromosomes has slightly different _ _.

cohesin, synaptonemal complex

The _ protein structure holds together sister chromatids within one chromosome, whereas the _ _ helps connect and align the two homologous chromosomes during synapsis

synaptonemal complex

The __ is a zipper-like protein structure found along the lengths of maternal and paternal homologous chromosomes, holding them closely aligned during synapsis and forming a tetrad.

synapsis

What is the process during prophase I of meiosis in which homologous chromosomes tightly pair and align to form a tetrad?

Prophase I

The process of crossing over occurs in _ _

crossing over

The process in which during prophase I, corresponding DNA segments/sections can be exchanged between nonsister chromatids of homologous chromosomes.

independent assortment

_ _ also occurs in Meiosis I, where each homologous pair can orient independently of other homologous pairs, producing different combinations of maternal and paternal chromosomes in gametes

copies, types, total

For 2n=6,

2 means that there are 2 _ of each chromosome type (one maternal and one paternal),

n means the number of chromosome _ in a complete set,

6 refers to the _ chromosome number

segregation

Mendel’s Law of _ states that two alleles of a gene separate during gamete formation, so each gamete only receives 1 allele

recessive

Random fertilization following allele separation allows two recessive alleles to reunite (aa), explaining the reappearance of the _ phenotype in approximately 25% of F2 offspring.

Independent assortment

Mendel’s Law of _ _ : During metaphase I, each homologous chromosome pair orients independently of other homologous pairs. Therefore, inheriting the maternal homolog of one chromosome type (A for example) does not determine whether a gamete receives the maternal (B) or paternal (b) homolog of another chromosome type.

Therefore, all 4 combinations are possible (AB Ab aB ab)

A, D

4. An Aa individual undergoes one normal meiosis, with no crossing over between the gene and its centromere. Which statements about the four resulting haploid cells are correct? Select all that apply.

A. Each cell carries one allele of this gene.

B. All four cells must carry A because A is dominant.

C. Each cell carries both A and a.

D. Two cells carry A and two carry a.

genetic linkage

occurs when two genes are located on the same chromosome and therefore tend to be inherited together, particularly when they are close to one another, as there is a lower chance of crossing over occurring between them

chromosome combinations

Independent assortment can create 2^n possible _ _ (as each homologous pair contains a chromosome from mom and dad)

homologous chromosome

n in 2^n for chromosome combinations represents the number of _ _ pairs

genotypes

2^h represents the possible distinct gamete _

heterozygous

h in 2^h represents the number of _ gene pairs

sister chromatid

Each _ _ represents one DNA molecule

20

After anaphase, if there are 10 chromosomes to begin with, how many are there? Remember that when the chromatids separate, each is considered its own chromosome

metaphase I

Mendel's Law of Independent Assortment: Alleles of genes on different chromosomes assort independently into gametes because homologous chromosome pairs orient independently during _-_

crossing over

2^n assumes that no _ _ occurs, and whole maternal or paternal homologs are inherited in gametes

homologous pair

Professor said that during normal Meiosis I, there is generally at least 1 cross over that occurs per _ _

combinations

In humans, the 23 chromosome pairs typically undergo numerous crossover events in total, making the possible genetic _ vastly greater than 2²³

chiasmata

Represent points where non-sister chromatids of homologous chromosomes physically connect and exchange DNA during Prophase I of meiosis

chiasmata

_ can occur at several different locations along each chromosome, resulting in lots and lots of different possible gametes

different

Each crossover involves two nonsister chromatids, but different crossover events can involve _ pairs of nonsister chromatids.

Prophase I, synaptonemal complex

Synapsis - include which phase: Homologous chromosomes pair closely, supported by the _ _

Prophase I, recombination proteins

Crossing over - include which phase: _ _ help exchange corresponding DNA segments between nonsister chromatids.

separase

(Include which phase): _ allows arm cohesin to be released in Anaphase I, so homologous chromosomes separate and chiasmata is released.

This enzyme also cleaves centromeric cohesin during anaphase II, allowing sister chromatids to separate

cohesin

_ holds together sister chromatids directly, not homologous chromosomes

P

The synaptonemal complex holds together non-sister chromatids (chromatids from different members of homologous pair). It forms and dissolves during _ _

A3, B2, C1

Mendel's observation	Chromosome behavior
A. Offspring inherit genetic information from both parents.	1 Homologous chromosomes separate during meiosis I.
B. Different genes can assort independently.	2 Different homologous chromosome pairs orient independently during metaphase I.
C. Two alleles segregate during gamete formation, allowing each gamete to receive one allele	3 Fertilization combines maternal and paternal chromosomes.

Match Mendel’s observation to the chromosome behavior (ex: A1…)

traits, factors, information

Mendel’s Understanding of Heredity vs. Modern Genetics

Imagine a pea plant with genotype Pp.

• Hereditary _ : Its flowers are purple.

• Hereditary _: It inherited two alleles, P and p, one from each parent.

• Hereditary _: The DNA sequences of those alleles contain instructions that influence flower pigmentation.

Morgan

embryologist and developmental biologist who became famous for his research on fruit fly genetics.

He began working with fruit flies at Columbia University, initially hoping to challenge Mendel's ideas.

he also later:

• later moved to Caltech and helped establish its biology department.

• He studied embryology and the development of organisms such as sea urchins.

• He helped establish marine biological research associated with Caltech.

• His fruit fly work eventually became foundational to genetics.

maintain, reproduce, offspring

Morgan used Drosophila melanogaster because they were relatively easy to _, _ quickly, and produce many _.

spontaneous

While Morgan examined Drosophila Melanogaster, he did not apply x-rays or chemical mutagens to create mutations, and rather looked for _ mutations (naturally-occurring), where he eventually found a white-eyed male

autosomal recessive

Morgan’s finding of his first cross, where it was between X^w+ X^w+ red female and x^w Y white male gave rise to the following progeny:

2 red females, 1 red male, and 1 white male; however, there were no white females.

If eye color were inherited as a simple _ _ trait, white-eyed offspring could be either male or female. But, Morgan observed that the white-eye phenotype appeared only in males in this cross. This suggested eye color was somehow connected to biological sex.

reciprocal cross

a cross that reverses which sex of parent carries a particular phenotype

x-linked

In Morgan’s studies, when the reciprocal (second) cross was taken, the F1 generation had 50/50 white and red progeny, whereas the F1 in the initial cross had 100% red-eyed progeny. Additionally, in the F2 offspring, 25% of white-eyed females were expected in the progeny, but 0% were expected in the initial cross. Finally, in the second cross, there was a 1:1:1:1 phenotypic expected ratio of progeny, whereas a 2:1:1 in the first cross.

Thus, reciprocal crosses can give different results when a gene is _ _

Nettie Stevens

a pioneering geneticist who helped establish the relationship between sex chromosomes and biological sex; helped establish the XX/XY sex-determination system by studying chromosomes in insects.

• Her work was crucial to understanding Morgan's results.

• Stevens studied germ cells and chromosomes in insects.

  She noticed that:

  • Females had matching chromosome pairs.

  • Males had a pair of sex chromosomes that looked different in size.

  • One was relatively large and the other smaller.

EB Wilson

cell biologist who studied sex chromosomes around the same period.

sex

Stevens proposed that chromosomes were associated with determining _

hemizygous

Most autosomal genes have two copies, one on each homologous chromosome inherited from the mother and father. Their alleles may be identical (AA) or different (Aa).

_ occurs when an otherwise diploid organism has only one copy of a particular gene.

For example, XY males are _ for many X-linked genes (white fruit fly gene) because these genes are present on their single X chromosome but have no corresponding copy on the Y chromosome.

hemizygosity

The condition in which an otherwise diploid organism has only one copy of a particular gene instead of the usual two, in autosomes and sex chromosomes

homologous

Two X chromosomes are _ , whereas X and Y chromosomes are only partially _

gene dosage

number of copies of a particular gene present in a cell or genome.

dosage compensation

organisms need mechanisms to compensate for differences in X-linked gene dosage

2, 1

X-linked gene dosage differs between XX and XY individuals because XX generally has _ copies of x-linked genes, and XY have _ for most X-linked genes

balance, complexes

Why could excess protein production be harmful?

It can disrupt protein _ and interfere with protein _ and cellular functions

protein complex

When 2 or more proteins are physically associated to perform a function

limiting

For example, imagine a protein complex requires:

- One molecule of protein A.

- One molecule of protein B.

If a cell produces 100 units of A and 100 units of B, the amounts are balanced.

But if it produces 200 units of A and only 100 units of B, the balance changes.

Excess protein A could interfere with normal cellular processes.

Gene B is the _ factor

X, X

To solve the problem of unequal X-linked gene dosage that can produce imbalanced gene products:

• Fruit flies: Increase expression from the single _ in XY males.

• Mammals: Silence most genes on one _ in XX females.

2, 1

With x-linked gene dosage, XX females tend to have _ copies of a particular X-linked gene, and XY males tend to have _ copy of that gene if there is no counterpart on the Y chromosome

mammals

One way dosage compensation occurs is in _, where most genes on one X chromosome are silenced in XX cells, so most X-linked genes are expressed from only one active X chromosome per cell.

transcription

Another way dosage compensation occurs is in fruit flies, where _ is increased from the single X chromosome in XY males to produce more X-linked gene products.

embryonic development

The process by which a fertilized egg (zygote) undergoes repeated cell divisions, growth, and cell differentiation to develop into a multicellular organism with specialized cells, tissues, and organs.