SBI3U · Unit 3 test review

Genetic Processes: Test Review

Everything for the Unit 3 test on one page: the key facts from all 6 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

DNA, Genes, Chromosomes and Mitosis

Nucleotide = phosphate + deoxyribose + base; A pairs with T and G pairs with C.
Gene: a DNA segment that codes for a protein. Chromosome: one DNA molecule with its proteins. Genome: all of an organism's DNA.
Cell cycle: G1 (growth) → S (DNA replication) → G2 (preparation) → M (mitosis and cytokinesis).
Mitosis: prophase, metaphase, anaphase, telophase (PMAT), then cytokinesis; it produces two diploid, genetically identical cells.
Somatic cells divide by mitosis for growth and repair; germ cells undergo meiosis to make gametes.
In a root-tip count, % of cells in a phase ≈ % of the cycle spent in that phase.

Meiosis

Diploid $2 n$ has two sets of chromosomes; haploid $n$ has one set. Humans: $2 n = 46$ , $n = 23$ .
Meiosis I separates homologous chromosomes (reduction division); meiosis II separates sister chromatids.
One diploid cell $\to$ four genetically different haploid cells.
Independent assortment gives $2^{n}$ chromosome combinations per gamete ( $2^{23} = 8388608$ in humans).
Crossing over happens between non-sister chromatids at chiasmata during prophase I.
Non-disjunction in meiosis I makes all four gametes abnormal; in meiosis II, two of the four.

Mendelian Inheritance

Law of segregation: the two alleles for a gene separate into different gametes, so each gamete carries one allele.
$A a \times A a$ gives genotypes $1 AA : 2 A a : 1 aa$ and phenotypes $3$ dominant $: 1$ recessive.
Test cross: unknown dominant $\times$ homozygous recessive. Any recessive offspring means the unknown is heterozygous; $A a \times aa$ gives $1 : 1$ .
Product rule (and): $P (A and B) = P (A) \times P (B)$ . Sum rule (or): $P (A or B) = P (A) + P (B)$ for mutually exclusive outcomes.
Genotype is the allele combination; phenotype is the observable trait.

Beyond Simple Dominance

Incomplete dominance: heterozygote is intermediate; $C^{R} C^{W} \times C^{R} C^{W}$ gives $1$ red $: 2$ pink $: 1$ white.
Codominance: both alleles are fully expressed in the heterozygote (roan cattle, type AB blood).
ABO: $I^{A}$ and $I^{B}$ are codominant, both dominant to $i$ ; type O is $ii$ .
X-linked recessive: males ( $X^{b} Y$ ) show the trait with one allele; females need $X^{b} X^{b}$ . Fathers pass their X to every daughter and never to sons.
$A a B b \times A a B b$ gives $9 : 3 : 3 : 1$ ; for independent genes, multiply the single-gene probabilities.

Gene Mutations and Single-Gene Disorders

A mutation is a permanent change in DNA base sequence; germline mutations are inherited, somatic mutations are not.
Substitution: one base replaced, changing at most one codon. Insertion or deletion of a number of bases that is not a multiple of 3 causes a frameshift.
Cystic fibrosis (CFTR gene, identified in 1989 by Lap-Chee Tsui's team in Toronto), sickle cell disease, PKU and Tay-Sachs are autosomal recessive; Huntington disease is autosomal dominant.
Heterozygote advantage: sickle cell carriers resist severe malaria, so the allele stays common where malaria is widespread.
Two carriers of a recessive allele (Aa × Aa) have a 1/4 chance of an affected child; an affected heterozygote with a dominant allele (Hh × hh) has a 1/2 chance.
Treatments range from diet (PKU) and symptom management to protein-targeting drugs, transplants, gene therapy, and medicines like insulin made by engineered organisms.

Chromosomal Disorders and Genetic Technologies

Trisomy (2n + 1) = 47 chromosomes in humans; monosomy (2n − 1) = 45. Both usually come from non-disjunction.
Down syndrome is trisomy 21; Turner syndrome is 45,X; Klinefelter syndrome is 47,XXY.
A karyotype arranges metaphase chromosomes in pairs by size, centromere position and banding pattern.
Recombinant DNA: restriction enzyme cuts, DNA ligase joins a gene into a plasmid, bacteria express the protein (e.g. human insulin).
Somatic gene therapy is not inherited; germline changes are, and Canadian law prohibits heritable changes to the human genome.
Embryonic stem cells are pluripotent; adult stem cells are multipotent; iPSCs are adult cells reprogrammed to be pluripotent.

Where marks get lost

Calling interphase a resting phase. The cell is growing, working and replicating its DNA during interphase. (DNA, Genes, Chromosomes and Mitosis)
Counting chromatids as chromosomes. After the S phase a human cell still has 46 chromosomes; each just has two sister chromatids (92 chromatids). (DNA, Genes, Chromosomes and Mitosis)
Thinking DNA replicates during mitosis. Replication happens in the S phase of interphase, before mitosis begins. (DNA, Genes, Chromosomes and Mitosis)
Mixing up the order of the middle phases. Chromosomes line up in metaphase (middle) and separate in anaphase (apart). (DNA, Genes, Chromosomes and Mitosis)
Saying mitosis halves the chromosome number. Mitosis keeps the number the same; only meiosis halves it. (DNA, Genes, Chromosomes and Mitosis)
Thinking meiosis II halves the chromosome number. The halving happens in meiosis I, when homologues separate; meiosis II separates sister chromatids and keeps the count at n. (Meiosis)
Counting an X-shaped replicated chromosome as two chromosomes. It is one chromosome with two sister chromatids; count centromeres, not arms. (Meiosis)
Mixing up homologous chromosomes with sister chromatids. Homologues come from different parents and can carry different alleles; sister chromatids are identical copies made by replication. (Meiosis)
Saying crossing over happens between sister chromatids. The exchange is between non-sister chromatids of a homologous pair; swapping identical copies would create nothing new. (Meiosis)
Assuming non-disjunction always affects all four gametes. That is true only for meiosis I; in meiosis II just one of the two cells is affected, so two gametes are normal. (Meiosis)
Writing gametes with two alleles, such as Rr in a Punnett square heading. Each gamete carries one allele per gene, because the alleles segregate in meiosis. (Mendelian Inheritance)
Reading 3 : 1 as a guarantee, for example saying "they already have one child with the recessive trait, so the next three will show the dominant trait." Each child is an independent event with the same 1/4 chance. (Mendelian Inheritance)
Mixing up the genotypic ratio (1 : 2 : 1) with the phenotypic ratio (3 : 1) when the question asks for one specifically. (Mendelian Inheritance)
Assuming a dominant allele is the more common allele in a population, or that it is better. Dominant describes only which allele shows in a heterozygote. (Mendelian Inheritance)
Concluding an unknown parent is homozygous after a test cross gives only a few offspring that all show the dominant trait. With a small sample, a heterozygous parent can produce only dominant offspring by chance. (Mendelian Inheritance)
Calling a pink snapdragon an example of codominance. Pink is an intermediate phenotype (incomplete dominance); codominance would show red and white patches or hairs side by side. (Beyond Simple Dominance)
Giving a male two alleles for an X-linked gene, such as writing $X^{B} Y^{b}$ . The Y chromosome does not carry the gene, so males have one allele. (Beyond Simple Dominance)
Reporting the probability for sons as a fraction of all children. "What fraction of sons" uses only the son boxes (out of 2); "what fraction of children" uses all 4 boxes. (Beyond Simple Dominance)
Listing dihybrid gametes with two alleles of the same gene, such as $Y y$ or $RR$ . Each gamete gets exactly one allele of each gene: $Y R$ , $Y r$ , $y R$ , $yr$ . (Beyond Simple Dominance)
Deciding a trait is dominant because it appears in many family members. Check the key clues instead: two unaffected parents with an affected child means recessive. (Beyond Simple Dominance)
Assuming every mutation is harmful. Many are silent or neutral, and some are beneficial, like the sickle allele in malaria regions. (Gene Mutations and Single-Gene Disorders)
Thinking any deletion causes a frameshift. Deleting exactly three bases, as in the most common CF mutation, removes one amino acid but keeps the reading frame. (Gene Mutations and Single-Gene Disorders)
Believing a somatic mutation, such as one caused by sunburn in a skin cell, can be passed to children. Only germline mutations are inherited. (Gene Mutations and Single-Gene Disorders)
Saying Huntington disease must be rare because it is dominant. Dominance describes how an allele is expressed, not how common it is; Huntington disease persists because symptoms usually start after people have had children. (Gene Mutations and Single-Gene Disorders)
Thinking a 1/4 chance means exactly one of every four children will be affected. Each child is a separate event with the same 1/4 chance, so a family could have zero, one or several affected children. (Gene Mutations and Single-Gene Disorders)
Saying Down syndrome is caused by a missing chromosome. It is an extra chromosome 21 (trisomy, 47 chromosomes). (Chromosomal Disorders and Genetic Technologies)
Treating a prenatal screening result as a diagnosis. Screening estimates a likelihood; a diagnostic test such as amniocentesis is needed to confirm a chromosomal condition. (Chromosomal Disorders and Genetic Technologies)
Assuming all gene therapy changes are passed to children. Somatic gene therapy alters only the patient's body cells; only germline changes are inherited. (Chromosomal Disorders and Genetic Technologies)
Thinking reproductive technologies always increase genetic diversity. Heavy use of a few sires and cloning can reduce diversity, while gene banks and importing genetics can increase it. (Chromosomal Disorders and Genetic Technologies)
Writing an ethics answer that only gives one opinion. An analysis should explain several perspectives and the evidence and values behind each. (Chromosomal Disorders and Genetic Technologies)

Practice test

24 questions from across the unit. Each one shows the lesson it comes from.

DNA, Genes, Chromosomes and Mitosis
1
Arrange the levels of genetic organization from smallest to largest.
1. 1Gene
2. 2Genome
3. 3Chromosome
4. 4Nucleotide
DNA, Genes, Chromosomes and Mitosis
2
A cell has its chromosomes lined up in a single row across its middle. Which phase is it in?
DNA, Genes, Chromosomes and Mitosis
3
A fruit fly cell has 2n = 8. How many chromatids are present in the cell at metaphase of mitosis? Give a whole number.
DNA, Genes, Chromosomes and Mitosis
4
What is the name for the division of the cytoplasm that follows mitosis?
Meiosis
5
In which stage do homologous chromosomes separate and move to opposite poles?
Meiosis
6
An organism has $2 n = 6$ . Ignoring crossing over, how many different chromosome combinations can its gametes have because of independent assortment?
combinations
Meiosis
7
What is the term for the failure of chromosomes or chromatids to separate properly during meiosis?
Meiosis
8
How many cells are produced when one diploid cell completes meiosis? Answer with a number.
Mendelian Inheritance
9
A pea plant has the genotype $Tt$ , where $T$ (tall) is dominant to $t$ (short). Which statement is correct?
Mendelian Inheritance
10
In guinea pigs, black fur ( $B$ ) is dominant to white fur ( $b$ ). Two $B b$ guinea pigs have 400 offspring over many litters. About how many offspring are expected to have white fur?
offspring
Mendelian Inheritance
11
A black guinea pig of unknown genotype is test-crossed with a white ( $bb$ ) guinea pig. Their litter has 3 black and 2 white offspring. What is the black parent's genotype?
Mendelian Inheritance
12
Match each term to its example (for pea seed shape, R = round, r = wrinkled).
Homozygous dominant genotype
Heterozygous genotype
Homozygous recessive genotype
Phenotype
Gamete from a heterozygote
Beyond Simple Dominance
13
A red bull and a white cow have a calf whose coat has a mix of red hairs and white hairs. What pattern of inheritance does this show?
Beyond Simple Dominance
14
Two parents are both $I^{A} i$ . What percentage of their children are expected to have type A blood?
%
Beyond Simple Dominance
15
What phenotypic ratio results from the test cross $A a B b \times aabb$ (complete dominance, independent assortment)?
Beyond Simple Dominance
16
Match each example to its pattern of inheritance.
Red and white snapdragons produce pink offspring
Type AB blood shows both A and B markers
A carrier mother passes a trait to half her sons
Two genes give a 9 : 3 : 3 : 1 phenotypic ratio
Gene Mutations and Single-Gene Disorders
17
Which change is most likely to alter every amino acid after the mutation site?
Gene Mutations and Single-Gene Disorders
18
A gene segment is read in codons starting at base 1. Base 14 is deleted. How many complete codons before the deletion are unaffected? Give a whole number.
Gene Mutations and Single-Gene Disorders
19
Put the events that lead to cystic fibrosis symptoms in order.
1. 1Mucus becomes thick and traps bacteria
2. 2Less water moves out of the cells by osmosis
3. 3Chloride ions cannot leave the cells lining the airways
4. 4Three bases are deleted from the CFTR gene
5. 5The CFTR protein is missing one amino acid and folds incorrectly
Gene Mutations and Single-Gene Disorders
20
SemBioSys Genetics of Calgary engineered which organism to produce human insulin?
Chromosomal Disorders and Genetic Technologies
21
How many chromosomes are in the somatic cells of a person with trisomy 18?
chromosomes
Chromosomal Disorders and Genetic Technologies
22
What is the name for a small circular piece of bacterial DNA used to carry a gene into a host cell?
Chromosomal Disorders and Genetic Technologies
23
Which kind of genetic change could be passed on to a patient's future children?
Chromosomal Disorders and Genetic Technologies
24
What is the name for a picture of an individual's chromosomes arranged in homologous pairs by size and banding pattern?

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

Generating a problem…