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Background information
Cycles Ch 3 – Sexual Reproduction in Humans
Human sexual reproduction
Learning outcomes

Recognise the process of fertilisation in the sexual reproduction of humans
In sexual reproduction, a new individual is formed when a male reproductive cell (male
gamete) fuses with a female reproductive cell (female gamete). The male produces gametes
called sperms, while the female produces gametes called eggs (ova). The fusion of a sperm
and an egg is called fertilisation. A fertilised egg is called a zygote. It will develop to form a
new individual with genes from both parents.
Fertilisation can take place internally or externally. Most aquatic animals, such as fish, fertilise
their eggs externally. A female deposits the eggs outside its body and a male releases
millions of sperms nearby. The sperms swim to the eggs and fuse with them. The fertilised
eggs develop outside the mother’s body.
The eggs of reptiles and birds are fertilised internally. A sperm fuses with an ovum while the
ovum is still inside the female’s body. The female lays the fertilised egg, which then develops
outside the mother’s body. For some animals, such as placental mammals, the egg is
fertilised internally and the fertilised egg develops inside the mother’s body until the offspring
is ready to be born.
The period from fertilisation to birth is known as the gestation period. During this period, the
zygote (fertilised egg) undergoes many changes. Different animals have different gestation
periods. In humans, it is about 40 weeks (about nine months).
The diagram below shows the male reproductive system of a human.
Bladder
Sperm duct
Urethra
Epididymis
Testis
Scrotum
Penis
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Background information
Sperms are produced in the testes. A sperm has a head with a large nucleus and a tail
(flagellum) which enables it to move by lashing about. The testes (singular: testis) are male
reproductive organs which produce sperms and sex hormones upon reaching puberty. The
scrotum holds the testes outside the body where they can be kept cool.
The sperms produced in the testes are stored temporarily in thinly coiled tubes, called epididymis,
before passing through the sperm ducts. Sex glands release fluids containing nutrients which
provide the sperms with energy to ‘swim’. The mixture of sperms with the fluids is called semen.
Semen leaves the penis through the urethra, which also transports urine from the bladder to the
penis at other times. The penis has erectile tissues. It becomes erect and ejects sperms and
deposits them in the vagina during sexual intercourse.
The diagram below shows the female reproductive system of a human.
Oviduct (or
Fallopian tube)
Ovary
Uterus
Cervix
Vagina
The ovaries are female reproductive organs which produce eggs (ova) and sex hormones. An
egg (ovum) has a ball-like structure and is larger than a sperm. Unlike a sperm, it does not
have a flagellum and cannot move by itself.
The ovaries of a female start releasing mature eggs upon reaching puberty. Usually, one
mature egg is released each month. The release of a mature egg from an ovary is called
ovulation. The mature egg passes through the oviduct (Fallopian tube) to the uterus (womb).
Fertilisation usually takes place in the oviduct. The walls of the oviduct contain muscles which
contract rhythmically to produce rippling movements. The inner walls are lined with hair-like
projections, called cilia. The egg is pushed by the cilia together with the rippling movements of
the oviduct towards the uterus.
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Background information
A uterus is a pear-shaped structure which has thick walls of muscles. It is where a foetus
develops during pregnancy. The uterus opens to a ring of muscles called the cervix, which is
the narrow neck of the uterus. The cervix widens during childbirth for the baby to come out of
the mother’s uterus through the vagina (birth canal). The vagina is a muscular tube where
sperms are deposited during sexual intercourse.
Of the millions of sperms deposited in the vagina, only a few hundred arrive at the oviduct. If
there is a mature egg in the oviduct, the sperms will cluster around it. Finally, one of the
sperms penetrates the egg and its nucleus fuses with that of the egg. Fertilisation has taken
place. The other sperms die soon after.
The fertilised egg (zygote) passes slowly along the oviduct towards the uterus, where its lining
continues to thicken. During this journey, the zygote divides repeatedly and forms a ball of
cells called an embryo. Upon reaching the uterus, the embryo attaches or implants itself to
the uterine lining, which is now thick and rich with blood vessels. The embryo is protected by
a fluid-filled sac, called an amniotic sac.
The cells of the embryo continue to divide many times to form new cells. The new cells
develop into different kinds of cells, tissues and organs. By about the 12th week, a foetus with
the main parts of the body and major organs is formed.
A foetus obtains nutrients from its mother’s blood through the placenta, while waste products,
such as carbon dioxide and urea, are passed into the mother’s blood to be removed. A
flexible tube, called the umbilical cord, carries blood between the placenta and the foetus.
(The belly button is where the umbilical cord was once attached to the body when the foetus
was in the mother’s womb.)
Umbilical cord
The foetus gradually grows and develops into a baby. About nine months after fertilisation has
taken place, the baby is ready to come out of the mother’s womb. The muscles of the uterus
start to contract and the amniotic sac breaks. The contractions increase in intensity and
frequency and the baby is pushed through the vagina and out of the mother’s body.
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Background information
Heredity
Learning outcomes

Show an understanding that living things reproduce to ensure continuity of their kind and
that many characteristics of an organism are passed on from parents to offspring
Reproduction is one of the characteristics of living things. Living things reproduce to ensure
that their own kind will continue to live on. Otherwise, they will become extinct.
During reproduction, the parents’ genes are passed on to their offspring through the sperm
and the egg. Since genes control certain traits or characteristics, these traits are passed on
from parents to offspring during reproduction. The transmission of biological traits through
genes from parents to offspring is known as heredity. Some inherited traits are controlled by a
single gene, while most others are controlled by more than one gene. The following are
examples of single-gene traits:

Tongue-rolling

Detached/attached ear lobes

Thumb-crossing

Widow’s peak

Hitchhiker’s thumb

Dimples
Alleles are different forms of the same gene. Different alleles produce variations in the
inherited traits. Each gene may have two alleles and the offspring inherits one allele from
each parent. If an organism has two copies of the same allele, it is homozygous for that trait.
If it has two different alleles, it is heterozygous. If an organism is heterozygous for a trait, one
allele may be dominant and is expressed, while the other may be recessive and is not
expressed. For example, the allele for detached ear lobe is dominant, while the allele for
attached ear lobe is recessive. An individual who is heterozygous for that trait will have
detached ear lobes as that allele is dominant. Someone who is homozygous dominant for the
trait will also have detached ear lobes. However, an individual who is homozygous recessive
for that trait will have attached ear lobes.
Internet links

Human sexual reproduction
http://www.pbs.org/wgbh/nova/miracle/program.html
http://www.justthefacts.org/clar.asp
http://www.howstuffworks.com/human-reproduction.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Sexual_Reproduction.html
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Background information
http://www.bbc.co.uk/schools/gcsebitesize/biology/variationandinheritance/1reproductiona
ndgenderrev5.shtml

Heredity
http://www.uni.edu/walsh/genetics.html
http://www.cccoe.net/genetics/index.html
http://www.dnaftb.org/dnaftb/1/concept/index.html
http://encarta.msn.com/encyclopedia_761564762/heredity.html
http://www.genetics.gsk.com/kids/index_kids.htm
Projects/Field trips

Conduct a class survey on the number of pupils with different genetic traits (e.g.
ability/inability to roll the tongue, attached/detached earlobes). Use a bar chart to plot the
results.

Conduct a case study on an extinct or endangered animal by finding out:
o
how it became extinct or endangered;
o
what can be done to help the endangered animal from becoming extinct;
o
what could have been done to prevent the animal from becoming extinct.
Recommended reading

The Usborne Internet–linked Introduction to Genes and DNA (Publisher: Usborne)

How the Body Works (Publisher: DK)

Eyewitness – Human Body (Publisher: DK)

Dorling Kindersley Eyewitness Guides – Life (Publisher: DK)
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