5.4.7 Sex Determination in Humans | 人类性别决定

📚 5.4.7 Sex Determination in Humans | 人类性别决定

In IGCSE Edexcel Science, understanding how biological sex is inherited helps explain the role of chromosomes, genes, and probability in reproduction. Humans have 23 pairs of chromosomes, and one of these pairs – the sex chromosomes – decides whether an individual develops as male or female. This topic links genetics to the practical use of Punnett squares and shows why the chance of having a boy or a girl is always close to 50%.

在IGCSE Edexcel科学课程中，理解生物性别如何遗传有助于解释染色体、基因和概率在生殖中的作用。人类有23对染色体，其中一对——性染色体——决定个体发育为男性还是女性。本主题将遗传学与庞尼特方格的实际应用联系起来，并说明为什么生男孩或女孩的概率总是接近50%。

1. Chromosomes and the Genetic Basis of Sex | 染色体与性别的遗传基础

Most human cells are diploid, containing 46 chromosomes arranged in 23 pairs. 22 of these pairs are autosomes (body chromosomes) that carry genes unrelated to sex. The 23rd pair consists of the sex chromosomes, labelled X and Y, which are responsible for determining biological sex. The combination of these sex chromosomes in the zygote dictates the developmental pathway.

大多数人类细胞是二倍体，含有46条染色体，组成23对。其中22对是常染色体（体染色体），携带与性别无关的基因。第23对是性染色体，标记为X和Y，它们负责决定生物性别。受精卵中性染色体的组合决定了发育路径。

2. Male and Female Karyotypes | 男性和女性的染色体核型

A typical female has two X chromosomes, written as 46,XX. A typical male has one X and one much smaller Y chromosome, written as 46,XY. The Y chromosome carries the SRY gene (sex-determining region Y), which triggers the development of testes. Without a Y chromosome, the embryo develops ovaries. This makes the Y chromosome the key determinant of maleness in mammals.

典型女性有两条X染色体，写作46,XX。典型男性有一条X染色体和一条小得多的Y染色体，写作46,XY。Y染色体上携带SRY基因（性别决定区Y基因），它触发睾丸的发育。如果没有Y染色体，胚胎发育出卵巢。这使得Y染色体成为哺乳动物雄性特征的关键决定因素。

3. Gametes and Meiosis | 配子与减数分裂

During gamete formation, meiosis halves the chromosome number so that each sperm or egg cell contains only 23 single chromosomes. All egg cells produced by a female carry one X chromosome because females are XX. In males, meiosis produces sperm cells that carry either an X chromosome or a Y chromosome in equal proportions – half the sperm are X-bearing and half are Y-bearing.

在配子形成过程中，减数分裂使染色体数目减半，因此每个精子或卵细胞只含有23条单体染色体。由于女性是XX，她产生的所有卵细胞都携带一条X染色体。男性通过减数分裂产生精子，这些精子携带X染色体或Y染色体，且比例相等——一半精子携带X染色体，一半携带Y染色体。

4. Fertilisation and Sex Determination | 受精与性别决定

At fertilisation, a sperm fuses with an egg to restore the diploid number. If an X-bearing sperm fertilises the egg, the zygote will be XX, developing into a female. If a Y-bearing sperm fertilises the egg, the zygote will be XY, developing into a male. The type of sperm that successfully reaches the egg therefore determines the sex of the offspring. The mother’s egg always contributes an X, so she has no influence on the baby’s sex.

受精时，精子与卵子结合，恢复二倍体数目。如果携带X染色体的精子使卵子受精，合子为XX，将发育为女性。如果携带Y染色体的精子使卵子受精，合子为XY，将发育为男性。因此，成功到达卵子的精子类型决定了后代的性别。母亲的卵子始终贡献一条X染色体，所以她对婴儿性别没有影响。

5. Using a Genetic Diagram (Punnett Square) | 使用遗传图解（庞尼特方格）

A Punnett square shows the random fusion of gametes and the expected ratios of offspring genotypes. For sex determination, we set the female parent’s gametes as X and X, and the male parent’s gametes as X and Y. The resulting table predicts that there is a 1 : 1 ratio of female (XX) to male (XY) offspring.

庞尼特方格展示配子的随机结合以及后代基因型的预期比例。对于性别决定，我们设母本配子为X和X，父本配子为X和Y。得到的表格预测女性（XX）与男性（XY）后代的比例为1 : 1。

Female Gametes: X X
Male Gametes: X Y

X (egg)	X (egg)
XX (female)	XX (female)
XY (male)	XY (male)

(Sperm: left column X and Y)

The Punnett square confirms that every pregnancy has a 50% probability of producing a girl and a 50% probability of producing a boy. This is a simple monohybrid cross where the sex chromosomes act as a single gene pair.

庞尼特方格确认，每次怀孕产生女孩的概率为50%，产生男孩的概率为50%。这是一个简单的单因子杂交，其中性染色体相当于一对等位基因。

6. The 50% Probability Rule | 50%概率规律

The ratio is theoretical: with a large number of births, the ratio of males to females tends to be very close to 1 : 1. In any single family, the observed ratio may differ due to chance. The key concept is that fertilisation is a random event, and previous children’s sexes do not influence the sex of future children.

该比例是理论上的：在有大量新生儿的情况下，男女比例非常接近1 : 1。在任何一个家庭中，观察到的比例可能因随机性而有所不同。关键概念是，受精是一个随机事件，之前孩子的性别不会影响未来孩子的性别。

7. Sex-Linked Inheritance – A Linked Concept | 性连锁遗传——相关概念

Although 5.4.7 focuses only on sex determination, genes located on the sex chromosomes show sex-linked inheritance. The X chromosome is larger and carries many genes, while the Y chromosome carries few genes besides the SRY gene. Conditions such as red-green colour blindness and haemophilia are caused by recessive alleles on the X chromosome. Males are more frequently affected because they have only one X chromosome, so a single recessive allele on it will express the trait.

虽然5.4.7只关注性别决定，但位于性染色体上的基因表现出性连锁遗传。X染色体较大，携带许多基因，而Y染色体除了SRY基因外携带的基因很少。红绿色盲和血友病等疾病是由X染色体上的隐性等位基因引起的。男性更容易患病，因为他们只有一条X染色体，所以上面只要有一个隐性等位基因就会表现性状。

8. Misconceptions About Sex Determination | 关于性别决定的常见误解

Myth: The mother’s diet or the timing of intercourse can determine the baby’s sex. Fact: Sex is determined purely by which sperm fertilises the egg; there is no scientific evidence that external factors alter the genetic mechanism.
Myth: Some families are “predisposed” to having only boys or only girls. Fact: Although parental chromosomal abnormalities can affect fertility, in typical XY/XX systems the probability remains 50% each pregnancy.
Myth: The sperm carrying Y chromosomes are faster, so more boys are conceived. Fact: Sperm speed and survival may differ slightly, but overall the birth ratio remains balanced.

同样中文解释：

误解：母亲的饮食或同房时间能决定婴儿性别。事实：性别纯粹由哪个精子使卵子受精决定，没有科学证据表明外部因素能改变遗传机制。
误解：某些家庭“注定”只生男孩或只生女孩。事实：虽然父母染色体异常可能影响生育能力，但在典型的XY/XX系统中，每次怀孕的概率仍然是50%。
误解：携带Y染色体的精子游得更快，所以怀男孩更多。事实：精子速度和存活率可能略有差异，但总体上出生比例保持平衡。

9. Real-World Data and the Human Sex Ratio | 现实数据和人类性别比

The global human sex ratio at birth is approximately 105 boys to 100 girls, meaning slightly more males are conceived and born. Scientists think this small deviation may be due to a slightly higher rate of Y-bearing sperm reaching the egg, or differential survival of male embryos. However, for IGCSE, you simply state that the expected ratio is 1 : 1, and real data show minor variations due to biological and statistical factors.

全球人类出生性别比约为105个男孩对100个女孩，这意味着受孕和出生的男孩稍多。科学家认为，这种微小偏离可能是由于携带Y染色体的精子到达卵子的几率略高，或者男性胚胎存活率略有不同。然而，在IGCSE考试中，你只需说明预期比例为1 : 1，真实数据因生物学和统计因素而存在微小差异。

10. Summary of Key Points | 重点总结

Human cells contain 23 pairs of chromosomes; the 23rd pair are the sex chromosomes.
Female genotype: XX. Male genotype: XY.
All eggs carry an X chromosome; sperm carry either X or Y.
Sex of offspring is determined at fertilisation by the sperm.
Punnett squares predict a 1 : 1 ratio of females to males.
The probability of having a boy or a girl is ½ (50%) each pregnancy, independent of previous births.
The Y chromosome carries the SRY gene responsible for male development.
Sex-linked genetic disorders are more common in males due to the single X chromosome.

重点总结中文：

人类细胞含有23对染色体；第23对是性染色体。
女性基因型：XX。男性基因型：XY。
所有卵子携带一条X染色体；精子携带X或Y染色体。
后代性别在受精时由精子决定。
庞尼特方格预测女性与男性的比例为1 : 1。
每次怀孕生男孩或女孩的概率均为½（50%），与之前的孩子性别无关。
Y染色体携带负责男性发育的SRY基因。
性连锁遗传病因只有一条X染色体而在男性中更常见。

11. Exam Tips for IGCSE Edexcel Science | IGCSE Edexcel科学考试技巧

When answering questions on sex determination, always use the correct genetic terms: genotype, phenotype, gamete, fertilisation, and Punnett square. Remember to state the parental genotypes (XX and XY), the gametes (X and X; X and Y), and construct the Punnett square clearly. Then explain that the ratio of XX to XY is 1 : 1, giving a 50% chance for each sex. If the question asks for probability, express it as a fraction (½), a percentage (50%), or a ratio (1 : 1). Avoid vague language like “equal chance” without genetic reasoning.

在回答性别决定的问题时，务必使用正确的遗传学术语：基因型、表现型、配子、受精和庞尼特方格。记住写出亲本基因型（XX和XY）、配子（X和X；X和Y），并清晰地构建庞尼特方格。然后解释XX与XY的比例为1 : 1，意味着每种性别各有50%的机会。如果问题要求概率，请用分数（½）、百分比（50%）或比率（1 : 1）表示。避免使用“机会相等”等模糊语言而不提供遗传学推理。

12. Beyond the Specification – Environmental Influences? | 考纲之外——环境影响？

In some reptiles, sex is determined by incubation temperature rather than chromosomes, but this is not part of the Edexcel IGCSE Science specification. It is useful to contrast temperature-dependent sex determination with our chromosomal system to appreciate the diversity of life. For the exam, stick firmly to the human XX/XY chromosomal mechanism and the random fusion of gametes.

在一些爬行动物中，性别由孵化温度而非染色体决定，但这不在Edexcel IGCSE科学考纲范围内。将温度依赖性性别决定与我们的染色体系统进行对比，有助于理解生命的多样性。对于考试，请严格坚持人类XX/XY染色体机制以及配子的随机结合。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

5.4.7 Sex Determination in Humans | 人类性别决定