Start by watching the video below. For a deeper dive into this material, complete this tutorial in our AP Bio 2.0 curriculum: Topic 5.6, Part 1: Sex Determination

1. Watch this video

2. Study this summary

1. Karyotypes

A karyotype is an image that shows the chromosomes in a human diploid cell, artificially paired up by size and staining pattern. In each of the pairs, one of the chromosomes is from the mother, and the other is from the father.

Here are two karyotypes. The one on the left is of a person who is chromosomally male. The one on the right is for chromosomally female person.

Male Karyotype	Female Karyotype

2. How Is Sex Determined in Humans and other Mammals?

In mammals, the chromosomes can be classified as autosomes or sex chromosomes. Whereas autosomes are pairs of chromosomes that are the same in both males and females, sex chromosomes differ between the sexes and determine biological sex.
Humans have 23 pairs of chromosomes:
- Pairs 1–22 are autosomes, the same in males and females.
- The 23rd pair are sex chromosomes:
  - Females: XX
  - Males: XY
The Y Chromosome’s Role:
- Contains the SRY gene, which triggers development of the testes during embryonic development.
- The testes secrete the hormone testosterone, which causes the body to develop male traits.
The Sex Chromosome in the Sperm Determines the Offspring’s Sex:
- Eggs always carry an X chromosome.
- Sperm can carry either an X or a Y chromosome:
  - If an X-carrying sperm fertilizes the X-carrying egg, the offspring will be female (XX)
  - If a Y-carrying sperm fertilizes the X-carrying egg, the offspring will be male (XY)
50:50 Sex Ratio:
- The equal chances of sperm carrying X or Y explains the near-equal male-to-female birth ratio in humans and other mammals.

2. How Is Sex Determined in Birds?

Birds also have chromosomal sex determination, but it works differently:
- Females: ZW chromosomes
- Males: ZZ chromosomes
In Birds, the Sex Chromosome in the Egg Determines the Sex of the Offspring
- Female eggs can carry either Z or W chromosomes.
- Male sperm only carries Z chromosomes:
  - Z sperm + Z egg = Male (ZZ)
  - Z sperm + W egg = Female (ZW)
Result: The sex ratio is also about 50:50 in birds.

3. Temperature-Dependent Sex Determination (in some Reptiles):

In some reptiles (like sea turtles, crocodiles, and tuataras), temperature during embryonic development determines sex:

Sea Turtles:
- Warmer temperatures during development → Females
- Cooler temperatures during development→ Males
Tuataras:
- Warmer temperatures → Males
- Cooler temperatures → Females
Crocodiles:
- Low and high temperatures → Females
- Intermediate temperatures → Males

4. Haplodiploidy in Ants, Bees, and Wasps:

In haplodiploid sex determination:

Males are haploid: They develop from unfertilized eggs (1 set of chromosomes).
Females are diploid: They develop from fertilized eggs (2 sets of chromosomes).
Genetic Relatedness:
- Female workers inherit 100% of their father’s genes and 50% of their mother’s genes. As a result, the workers (who are all sisters) share an average of 75% of their genes. That makes the workers more closely related to one another than mammalian siblings are (75% relatedness in haplodiploid animals, vs. 50% in mammals).
- This high relatedness may explain their cooperation to support the queen and colony (a topic further explored in AP Bio Unit 8).

3. Master these flashcards

[qdeck qrecord_id=”sciencemusicvideosMeister1961-Sex Determination Flashcards, APBVP”]
[h] Sex Determination

[q] What are autosomes?
[a] Pairs of chromosomes that are the same in both males and females.

[q] What are sex chromosomes?
[a] Chromosomes that differ between the sexes and determine biological sex.

[q] How many pairs of chromosomes do humans have?
[a] 23 pairs of chromosomes.

[q] What are the first 22 pairs of human chromosomes called, and are they the same in males and females?
[a] They are called autosomes and are the same in males and females.

[q] What is the 23rd pair of human chromosomes called?
[a] The sex chromosomes.

[q] What sex chromosomes do females and males have?
[a] Females: XX, Males: XY.

[q] What does the SRY gene on the Y chromosome do?
[a] It triggers testes development and later testosterone production, leading to male traits.

[q] Which parent’s gamete determines the sex of the offspring in humans and other mammals?
[a] The sperm from the male parent.

[q] Why is the male-to-female birth ratio close to 50:50 in mammals?
[a] There’s a 50/50 chance of a sperm carrying an X or a Y chromosome.

[q] How is sex determined in birds?
[a] By the sex chromosomes: Females are ZW, and males are ZZ.

[q] Which parent’s gamete determines the sex of the offspring in birds?
[a] The egg from the female parent.

[q] Why is the male-to-female birth ratio close to 50:50 in birds?
[a] There’s a 50/50 chance of an egg carrying Z or a W chromosome.

[q] What determines the sex of some reptiles?
[a] Temperature during embryonic development.

[q] What is haplodiploidy?
[a] A system of sex determination where males are haploid (with one set of chromosomes) and females are diploid (with two sets of chromosomes).

[q] How do males develop in haplodiploid species like ants, bees, and wasps?
[a] From unfertilized eggs.

[q] How do females develop in haplodiploid species like ants, bees, and wasps?
[a] From fertilized eggs.

[q] Why are female worker bees more closely related to their sisters than mammal siblings are?
[a] They share, on average, 75% of their genes because they inherit 100% of their father’s genes and, on average, half of their mother’s genes.

[/qdeck]

4. Tackle these quizzes

4.1 Sex Determination in Mammals

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-Sex Determination in Mammals, APBVP”]

[h]Sex Determination in Mammals

[i]

[q] In the diagram below, the father’s germ cell is represented by

[textentry single_char=”true”]

[c]ID E=[Qq]

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[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, the mother’s germ cell is represented by

[textentry single_char=”true”]

[c]ID I=[Qq]

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[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, the sperm cell that will produce a female is represented by

[textentry single_char=”true”]
[c]ID M=

Cg==[Qq]

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[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, the sperm cell that will produce a male is represented by

[textentry single_char=”true”]

[c]ID Q=[Qq]

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[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, an unfertilized egg cell is represented by

[textentry single_char=”true”]
[c]ID U=

Cg==[Qq]

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[c]IEVudGVyIHdvcmQ=[Qq]

[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, a female zygote is represented by

[textentry single_char=”true”]

[c]ID Y=[Qq]

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[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, a male zygote is represented by

[textentry single_char=”true”]

[c]ID c=[Qq]

[f]IENvcnJlY3QhIEEgbWFsZSB6eWdvdGUgaXMgcmVwcmVzZW50ZWQgYnkgJiM4MjIwOzcuJiM4MjIxOw==[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IE5vLg==[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, sperm “1” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c]IF k=[Qq]

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[c]ICo=[Qq]

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[q] In the diagram below, sperm “2” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c]IF g=[Qq]

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[c]IEVudGVyIHdvcmQ=[Qq]

[c]ICo=[Qq]

[q] In the diagram below, the eggs at “3” or “4” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c]IF g=[Qq]

[f]IENvcnJlY3QuIFVuZmVydGlsaXplZCBlZ2dzIGNhbiBvbmx5IGNhcnJ5IGFuIFggY2hyb21vc29tZS4=[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[c]ICo=[Qq]

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[q] In the diagram below, which number refers to a cell or organism that would have an X and a Y chromosome?

[textentry single_char=”true”]

[c]ID U=[Qq]

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[c]IEVudGVyIHdvcmQ=[Qq]

[c]ICo=[Qq]

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[x][restart]

[/qwiz]

4.1 Sex Determination in Humans and Other Animals

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Sex Determination in Humans and Other Animals, APBVP”]

[h] Sex Determination in Humans and Other Animals

[q multiple_choice=”true”] In these animals, sex is determined by the inheritance of a specific sex-inducing chromosome that comes from the male gamete.

[c]IGJpcmRz[Qq]

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[c]IG1hbW 1hbHM=[Qq]

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[c]IGhvbmV5YmVl[Qq]

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[q multiple_choice=”true”] In these animals, sex is determined by the inheritance of a specific sex-inducing chromosome that comes from the female gamete.

[c]IGJp cmRz[Qq]

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[q multiple_choice=”true”] In these animals, sex is determined by the temperature at which the eggs develop.

[c]IGJpcmRz[Qq]

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[c]IGhvbmV5YmVlcw==[Qq]

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[q multiple_choice=”true”] In these animals, males develop from unfertilized eggs; females develop from fertilized eggs.

[c]IGJpcmRz[Qq]

[c]IG1hbW1hbHM=[Qq]

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[c]IGhvbmV5 YmVlcw==[Qq]

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[q multiple_choice=”true”] The diagram below shows a female and a male fruit fly. Below the flies, you can see the sex chromosomes associated with each sex. Based on the diagram, you can predict that

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[f]IE5vLiBJbiBob25leWJlZXMsIG1hbGVzIGFyZSBoYXBsb2lkLCBhbmQgZmVtYWxlcyBhcmUgZGlwbG9pZC4gVGhlIGZhY3QgdGhhdCBhIG1hbGUgZnJ1aXQgZmx5IGhhcyBib3RoIGFuIFggYW5kIGEgWSBjaHJvbW9zb21lIGluZGljYXRlcyB0aGF0IG1hbGUgZnJ1aXQgZmxpZXMgYXJlIGRpcGxvaWQuIEhlcmUmIzgyMTc7cyBhIGhpbnQgZm9yIG5leHQgdGltZTogd2hhdCBhbmltYWxzIGRvIHlvdSBrbm93IGFib3V0IGluIHdoaWNoIHRoZSBmZW1hbGVzIGFyZSBYWCBhbmQgdGhlIG1hbGVzIGFyZSBYWT8=[Qq]

[c]IFNleCBkZXRlcm1pbmF0aW9uIGluIGZydWl0IGZsaWVzIHdvcmtzIGFzIGl0IGRvZXMgaW4gYmlyZHMsIHdpdGggdGhlIGZlbWFsZSBwcm9kdWNpbmcgYSBnYW1ldGUgdGhhdCBkZXRlcm1pbmVzIHRoZSBzZXggb2YgdGhlIG9mZnNwcmluZy4=[Qq]

[f]IE5vLiBJbiBiaXJkcywgdGhlIGZlbWFsZSBoYXMgdHdvIGRpZmZlcmVudCBzZXggY2hyb21vc29tZXMsIFogYW5kIFcuIEluIGZydWl0IGZsaWVzLCB0aGUgZmVtYWxlIGhhcyB0d28gWCBjaHJvbW9zb21lcy4gVGhlcmVmb3JlLMKgIHRoZSBmZW1hbGUgY2FuIG9ubHkgcGFzcyBvbiBhbiBYIGNocm9tb3NvbWUuIEhlcmUmIzgyMTc7cyBhIGhpbnQgZm9yIG5leHQgdGltZTogd2hhdCBhbmltYWxzIGRvIHlvdSBrbm93IGFib3V0IGluIHdoaWNoIHRoZSBmZW1hbGVzIGFyZSBYWCBhbmQgdGhlIG1hbGVzIGFyZSBYWT8=[Qq]

[c]IHNleCBkZXRlcm1pbmF0aW9uIGlzIHRlbXBlcmF0dXJlIGRlcGVuZGVudC4=[Qq]

[f]IE5vLiBJbiB0ZW1wZXJhdHVyZS1kZXBlbmRlbnQgc2V4IHNlbGVjdGlvbiwgdGhlcmUmIzgyMTc7cyBubyBjaHJvbW9zb21hbCBkaWZmZXJlbmNlIGJldHdlZW4gbWFsZXMgYW5kIGZlbWFsZXMuIEhlcmUmIzgyMTc7cyBhIGhpbnQgZm9yIG5leHQgdGltZTogd2hhdCBhbmltYWxzIGRvIHlvdSBrbm93IGFib3V0IGluIHdoaWNoIHRoZSBmZW1hbGVzIGFyZSBYWCBhbmQgdGhlIG1hbGVzIGFyZSBYWT8=[Qq]

[c]IFNleCBkZXRlcm1pbmF0aW9uIGluIGZydWl0IGZsaWVzIHdvcmtzIGFzIGl0IGRvZXMgaW4gbWFtbWFscywgd2l0aCB0 aGUgbWFsZSBwcm9kdWNpbmcgYSBnYW1ldGUgdGhhdCBkZXRlcm1pbmVzIHRoZSBzZXggb2YgdGhlIG9mZnNwcmluZy4=[Qq]

[f]IENvcnJlY3QhIE1hbGUgZnJ1aXQgZmxpZXMgaGF2ZSBhbiBYIGFuZCBhIFkgY2hyb21vc29tZS4gQXMgaW4gbWFtbWFscywgWFkgbWFsZXMgcHJvZHVjZSBzcGVybSB0aGF0IGhhdmUgZWl0aGVyIGFuIFggb3IgYSBZIGNocm9tb3NvbWUuIFgtYmVhcmluZyBzcGVybSByZXN1bHQgaW4gZmVtYWxlIChYWCkgb2Zmc3ByaW5nLiBZLWJlYXJpbmcgc3Blcm0gcmVzdWx0IGluIG1hbGUgKFhZKSBvZmZzcHJpbmcu[Qq]

[q] In the diagram below, which letter indicates the temperature at which the majority of the sea turtles hatchlings will be female?

[textentry single_char=”true”]

[c]IE I=[Qq]

[f]IE5pY2U6IGF0IGhpZ2ggdGVtcGVyYXR1cmVzLCB0aGUgbWFqb3JpdHkgb2Ygc2VhIHR1cnRsZSBoYXRjaGxpbmdzIHdpbGwgYmUgZmVtYWxlLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBTdHVkeSB0aGUgZGlhZ3JhbSBvbiB0aGUgbGVmdC4gVGhlIFgtYXhpcyBzaG93cyBpbmNyZWFzaW5nIHRlbXBlcmF0dXJlLCBhbmQgdGhlIFktYXhpcyBzaG93cyBhbiBpbmNyZWFzaW5nIHByb3BvcnRpb24gb2YgbWFsZSBoYXRjaGxpbmdzLiBXaGVyZSBpcyB0aGUgcHJvcG9ydGlvbiBvZiBtYWxlcyB0aGUgbG93ZXN0IChhbmQsIHRoZXJlZm9yZSwgdGhlIHByb3BvcnRpb24gb2YgZmVtYWxlcyB0aGUgaGlnaGVzdCk/[Qq]

[c]IEVudGVyIGxldHRlcg==[Qq]

[q multiple_choice=”true”]Among Tuataras, the majority of hatchlings will be male at

[c]bG93IHRlbXBlcmF0dXJlcw==[Qq]

[f]Tm8uIFN0dWR5IHRoZSBncmFwaCBpbiB0aGUgY2VudGVyLiBUaGUgWC1heGlzIHNob3dzIGluY3JlYXNpbmcgdGVtcGVyYXR1cmUsIGFuZCB0aGUgWS1heGlzIHNob3dzIGFuIGluY3JlYXNpbmcgcHJvcG9ydGlvbiBvZiBtYWxlIGhhdGNobGluZ3MuIEF0IHdoYXQgdGVtcGVyYXR1cmUgaXMgdGhlIHByb3BvcnRpb24gb2YgbWFsZSBoYXRjaGxpbmdzIHRoZSBoaWdoZXN0Pw==[Qq]

[c]bWVkaXVtIHRlbXBlcmF0dXJlcw==[Qq]

[c]aGlnaCB0ZW1w ZXJhdHVyZXM=[Qq]

[f]TmljZSBqb2IuIEF0IGxldHRlciAmIzgyMjA7RCYjODIyMTsgdGhlIG1ham9yaXR5IG9mIGhhdGNobGluZ3Mgd2lsbCBiZSBtYWxlLCBhbmQgdGhpcyBjb3JyZXNwb25kcyB3aXRoIGhpZ2ggdGVtcGVyYXR1cmVzLg==[Qq]

[q]In the diagram below, which letter indicates the temperature at which the majority of the crocodilian hatchlings will be male?

[textentry single_char=”true”]

[c]Rg ==[Qq]

[f]IE5pY2U6IGF0IG1lZGl1bSB0ZW1wZXJhdHVyZXMgKGxldHRlciBGKSwgdGhlIG1ham9yaXR5IG9mIGNyb2NvZGlsaWFuIGhhdGNobGluZ3Mgd2lsbCBiZSBtYWxlLg==[Qq]

[c]ICo=[Qq]

[c]IEVudGVyIGxldHRlcg==[Qq]

[q]In the diagram below, the chromosome that’s responsible for the creation of female offspring is best represented by number

[textentry single_char=”true”]

[c]NQ ==[Qq]

[f]IEF3ZXNvbWUuICYjODIyMDs1JiM4MjIxOyBzaG93cyBhICYjODIyMDtXJiM4MjIxOyBjaHJvbW9zb21lIGluIGFuIHVuZmVydGlsaXplZCBlZ2cuIFdoZW4gdGhhdCBlZ2cgaXMgZmVydGlsaXplZCwgdGhlIHJlc3VsdCB3aWxsIGJlIGEgWlcgenlnb3RlLCB3aGljaCB3aWxsIGRldmVsb3AgaW50byBhIGZlbWFsZS4=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBJbiBiaXJkcywgdGhlIFpaIHBhdHRlcm4gcmVzdWx0cyBpbiBhIG1hbGUsIGFuZCBaVyByZXN1bHRzIGluIGEgZmVtYWxlLiBUaGVyZWZvcmUsIG9ubHkgYW4gdW5mZXJ0aWxpemVkIGVnZyB3aXRoIGEgJiM4MjIwO1cmIzgyMjE7IGNocm9tb3NvbWUgd2lsbCBnaXZlIHJpc2UgdG8gYSBmZW1hbGUuIFdoZXJlIGNhbiB5b3UgZmluZCB0aGF0IGVnZz8=[Qq]

[c]IEVudGVyIGxldHRlcg==[Qq]

[q]In the diagram below, the queen is represented by number

[textentry single_char=”true”]

[c]Mw ==[Qq]

[f]IEF3ZXNvbWUuICYjODIyMDszJiM4MjIxOyBpcyBhIGRpcGxvaWQgb3JnYW5pc20gdGhhdCB1bmRlcmdvZXMgbWVpb3NpcyB0byBjcmVhdGUgNCBoYXBsb2lkIGdhbWV0ZXMuIChhbmQgd2hpY2ggYWxzbyByZWNlaXZlcyByb3lhbCBqZWxseSwgaW5kaWNhdGVkIGJ5ICYjODIyMDtDLiYjODIyMTspLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBMb29rIGZvciBhIGRpcGxvaWQgb3JnYW5pc20gdGhhdCB1bmRlcmdvZXMgbWVpb3NpcyB0byBjcmVhdGUgNCBoYXBsb2lkIGdhbWV0ZXMu[Qq]

[c]IDQ=[Qq]

[f]Tm8uIEl0IGNhbiYjODIxNzt0IGJlICYjODIyMDs0JiM4MjIxOyBiZWNhdXNlICYjODIyMDs0JiM4MjIxOyBuZXZlciBwcm9kdWNlcyBhbnkgZWdncyAod2hpY2ggbWVhbnMgdGhhdCBpdCBoYXMgdG8gYmUgYSBzdGVyaWxlIHdvcmtlcikuIExvb2sgZm9yIGFuIG9yZ2FuaXNtIHRoYXQmIzgyMTc7cyBib3RoIGRpcGxvaWQgYW5kIGVnZy1sYXlpbmcu[Qq]

[q]In the diagram below, meiosis is represented by the letter

[textentry single_char=”true”]

[c]QQ ==[Qq]

[f]IEV4Y2VsbGVudC4gJiM4MjIwO0EmIzgyMjE7IGluZGljYXRlcyBhIHByb2Nlc3MgYnkgd2hpY2ggYSBkaXBsb2lkIG9yZ2FuaXNtIChhdCAzKSBjcmVhdGVzIDQgaGFwbG9pZCBjZWxscyAoYXQgJiM4MjIwOzEmIzgyMjE7KS4gVGhhdCBwcm9jZXNzIGhhcyB0byBiZSBtZWlvc2lzLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBMb29rIGZvciBhIHByb2Nlc3MgYnkgd2hpY2ggYSBkaXBsb2lkIG9yZ2FuaXNtIChhdCAzKSBjcmVhdGVzIDQgaGFwbG9pZCBjZWxscyAoYXQgJiM4MjIwOzEmIzgyMjE7KS4=[Qq]

[q]In the diagram below, the process by which gametes are created by mitosis is represented by the letter

[textentry single_char=”true”]

[c]Qg ==[Qq]

[f]IEV4Y2VsbGVudC4gJiM4MjIwO0ImIzgyMjE7IHNob3dzIHR3byBoYXBsb2lkIHNwZXJtIGNyZWF0ZWQgYnkgYSBoYXBsb2lkIG9yZ2FuaXNtLiBUaGUgcHJvY2VzcyBvZiBjcmVhdGluZyB0aGF0IHNwZXJtIGhhcyB0byBiZSBtaXRvc2lzLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBMb29rIGZvciBhIGhhcGxvaWQgc3Blcm0gY2VsbCB0aGF0IGNvbWVzIGZyb20gYSBoYXBsb2lkIHBhcmVudC4=[Qq]

[q]In the diagram below, a haploid male drone is represented by the number

[textentry single_char=”true”]

[c]NQ ==[Qq]

[f]IEV4Y2VsbGVudC4gJiM4MjIwOzUmIzgyMjE7IG9uIHRoZSB0b3Agc2hvd3MgYSBoYXBsb2lkIHBhcmVudCBjcmVhdGluZyBoYXBsb2lkIHNwZXJtIGNlbGxzLsKgICYjODIyMDs1JiM4MjIxOyBvbiB0aGUgYm90dG9tIHNob3dzIGFuIG9yZ2FuaXNtIHRoYXQgZGV2ZWxvcHMgZnJvbSBhbiB1bmZlcnRpbGl6ZWQgZWdnLiBCb3RoIG9mIHRoZXNlIHByb2Nlc3NlcyBhcmUgYXNzb2NpYXRlZCB3aXRoIGRyb25lcy4=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBMb29rIGZvciBhIGhhcGxvaWQgcGFyZW50IGNyZWF0aW5nIGhhcGxvaWQgc3Blcm0gY2VsbHMsIG9yIGFuIG9yZ2FuaXNtIHRoYXQgZGV2ZWxvcHMgZnJvbSBhbiB1bmZlcnRpbGl6ZWQgZWdnLg==[Qq]

[q]The sex-determination system shown below is [hangman] [hangman] sex determination.

[c]dGVtcGVyYXR1cmU=[Qq]

[c]ZGVwZW5kZW50[Qq]

[q]In the sex-determination system shown below, the males are [hangman] and the females are [hangman]. The entire system is called [hangman] sex determination.

[c]aGFwbG9pZA==[Qq]

[c]ZGlwbG9pZA==[Qq]

[c]aGFwbG9kaXBsb2lk[Qq]

[q]In both birds and mammals, a specific [hangman] determines whether the offspring will be male or female. But while in birds it’s the [hangman] parent who provides the sex-determining gamete, in mammals it’s the [hangman] parent.

[c]Y2hyb21vc29tZQ==[Qq]

[c]ZmVtYWxl[Qq]

[c]bWFsZQ==[Qq]

[/qwiz]

What’s Next?

Proceed to the next tutorial: Nondisjunction