Bio Principles Unit 4 : lecuture 3

What was Mendel's major contribution to genetics?

He discovered patterns of inheritance through mathematical ratios, without knowledge of chromosomes.

When was the chromosome theory of inheritance developed?

Around 1900, when scientists connected Mendel’s work to chromosomes and meiosis.

What is the chromosome theory of inheritance?

It links Mendel's laws to chromosomes, placing genes at specific loci on chromosomes.

What determines whether two genes will be inherited together?

Their physical proximity on the same chromosome.

What breaks the inheritance of linked genes?

Crossing over during meiosis.

Why are red hair and freckles often inherited together?

The genes for these traits are closely linked on the same chromosome.

Do linked genes follow the law of independent assortment?

No, they are inherited as a package unless separated by crossing over.

What happens if genes A and B are linked on a chromosome?

They are inherited together in gametes unless recombination occurs.

How do you form gametes from an individual heterozygous for linked genes (e.g., AaBb with A and B linked)?

Only two gamete types: AB and ab, each at 50%.

When are recombination events more likely to occur between genes?

When the genes are farther apart on the chromosome.

What is the recombination frequency between genes a measure of?

The physical distance between them on the chromosome.

What is a linkage map?

A genetic map based on recombination frequencies between genes.

What organism was used to create early linkage maps?

Drosophila melanogaster (fruit fly).

What is a model organism?

An organism extensively studied and genetically mapped to model biological processes.

Why are mice considered good model organisms?

They share genetic similarity with humans and allow for genetic experimentation.

What is genetic recombination?

The exchange of genetic material between homologous chromosomes during meiosis.

How is sex determined in humans?

By the presence of X and Y chromosomes: XX = female, XY = male.

How is sex determined in sea turtles?

By the incubation temperature of the eggs.

What is the rule of thumb for sea turtle sex determination?

Hot chicks, cool dudes — warmer eggs become females, cooler eggs become males.

What is the XY sex determination system?

Males are XY, females are XX — found in humans and Drosophila.

What is the XO sex determination system in insects?

Females are XX, males have only one X (no Y).

What is the ZW sex determination system in birds like chickens?

Females are ZW, males are ZZ.

How does the haplodiploid system determine sex in bees and ants?

Females are diploid, males are haploid.

What is nondisjunction?

The failure of chromosomes to separate properly during meiosis.

What syndrome is caused by trisomy 21?

Down syndrome.

What causes Down syndrome?

Nondisjunction during meiosis resulting in an extra chromosome 21.

What is Turner syndrome?

A condition where a female has only one X chromosome (XO), causing sterility and physical abnormalities.

What is Klinefelter syndrome?

A condition where a male has two Xs and one Y (XXY), often leading to sterility and low testosterone.

What was wrongly concluded about XYY males?

That they are more aggressive and likely to be criminals, based on faulty correlation.

What is a wild type in genetics?

The normal, non-mutated phenotype, often labeled with a "+".

What is a mutant phenotype?

Any phenotype that differs from the wild type due to mutations.

Which experiment helped establish X-linked inheritance?

Morgan's experiment with Drosophila eye color.

1. What was the wild type eye color in Morgan's Drosophila experiment?

Red

2. What eye color was considered the mutant in Morgan's experiment?

White

3. What result did Morgan get when he crossed a red-eyed female with a white-eyed male?

All offspring had red eyes

4. What did Morgan observe in the F2 generation that suggested sex linkage?

All females had red eyes, but half the males had white eyes

5. What chromosome carries the gene for eye color in Morgan’s experiment?

The X chromosome

6. Why are males more affected by X-linked traits than females?

Males have only one X chromosome

7. What determines the sex of the offspring in humans?

The sperm

8. What type of gametes do females produce regarding sex chromosomes?

All eggs carry an X chromosome

9. What type of gametes do males produce regarding sex chromosomes?

50% carry X and 50% carry Y

10. Why do you always include the Y chromosome in genetic crosses involving sex-linked traits?

To determine the sex of the offspring

11. What is the wild-type allele symbol for Drosophila eye color?

X^W⁺

12. What is the mutant allele symbol for Drosophila eye color?

X^w

13. Can males be carriers for X-linked traits?

No, they either have the trait or they don't

14. What is the mode of inheritance for colorblindness?

X-linked recessive

15. What is the mode of inheritance for hemophilia?

X-linked recessive

16. Who passes the X-linked condition to their sons?

The mother

17. What chromosome does a son inherit from his father?

Y chromosome

18. What does a capital H represent in hemophilia inheritance problems?

Normal allele

19. What does a lowercase h represent in hemophilia inheritance problems?

Hemophilia allele

20. Why can females be carriers but not affected by X-linked recessive diseases?

They have two X chromosomes and one may carry the normal allele

21. What is a Barr body?

An inactivated X chromosome in females

22. Why do calico cats have patches of black and orange fur?

Random X-inactivation (Barr bodies) expressing different fur color alleles

23. What type of inheritance causes the calico pattern in cats?

X-linked inheritance with random X-inactivation

24. Can males be calico?

Very rarely, and only if they have two X chromosomes (XXY)

25. What is genomic imprinting?

Silencing of certain genes in one parent's chromosome during embryonic development

26. Does genomic imprinting happen on sex chromosomes or autosomes?

Autosomes

27. Can imprinting turn off either maternal or paternal alleles?

Yes, depending on the gene and which parent it came from

28. What is the effect of genomic imprinting on gene expression?

Only one allele is expressed while the other is silenced

29. What is an example condition caused by imprinting mentioned in the lecture?

Dwarfism in mice

30. Are there known Y-linked genetic diseases?

Very few, mostly related to male sex determination (e.g., SRY gene)

**What is a virus made of?

A protein capsule and DNA or RNA.**

**Are viruses considered living outside of a host cell?

No, they are non-living outside of cells.**

**What do viruses inject into host cells?

Their genetic material (DNA or RNA).**

**What happens to a host cell after viral injection?

It becomes a factory for viral protein and genetic material, eventually bursts (lyses).**

**What did scientists learn from viruses about DNA?

That DNA could be the molecule responsible for heredity.**

**Who discovered patterns of inheritance through pea plant experiments?

Mendel.**

**Who discovered that genes are on chromosomes?

Morgan.**

**Who conducted the first transformation experiment?

Griffith.**

**What bacteria did Griffith use in his experiment?

Streptococcus pneumoniae (smooth "S" strain and rough "R" strain).**

**What happened when Griffith injected heat-killed S strain and live R strain?

The mouse died because live R bacteria were transformed into pathogenic S type.**

**What is bacterial transformation?

The process by which bacteria take up foreign DNA from their environment.**

**What structure allows bacteria to exchange genetic material?

Pili (bridge-like structures).**

**What molecule was later identified as responsible for transformation?

DNA.**

**What did Chargaff discover about DNA?

The amount of adenine equals thymine, and cytosine equals guanine.**

**What does Chargaff’s Rule suggest about base pairing?

A pairs with T, and C pairs with G.**

**If a DNA molecule has 31% adenine, how much thymine is present?

31%.**

**If adenine and thymine make up 62% of DNA, what percent is cytosine?

19%.**

**Who captured the first X-ray image of DNA structure?

Rosalind Franklin.**

**What structure did Franklin’s X-ray suggest?

A helical structure (double helix).**

**Who used Franklin’s data to build the DNA model?

Watson and Crick.**

**What bonds hold DNA strands together?

Hydrogen bonds between base pairs.**

**How many hydrogen bonds are between A and T?

Two.**

**How many hydrogen bonds are between C and G?

Three.**

**What does anti-parallel mean in DNA structure?

The two strands run in opposite directions (5' to 3' and 3' to 5').**

**What end of DNA has a phosphate group?

The 5' (5-prime) end.**

**What end of DNA has a hydroxyl group?

The 3' (3-prime) end.**

**What are the components of a DNA nucleotide?

A phosphate, a deoxyribose sugar, and a nitrogenous base.**

**What is the shape of the DNA molecule?

A double helix.**