My Science School

 

 

 

What are X and Y chromosomes?

                         The 23rd pair of genes determines the sex of a baby. These are X and Y chromosomes. A woman has two X chromosomes, while a man has one X and one Y chromosome. When the chromosomes join together in fertilization, if the sperm contains an X chromosome, it will combine with the X chromosome of the female to produce XX — a girl. If the sperm contributes the Y chromosome, the baby will have one X and one Y chromosome and will be XY — a boy.

 

 

 

 

 

What is a mutation?

                        Mutations are changes in the DNA inside a cell that affect the genes and chromosomes. Mutations arise because of errors in the DNA molecule when it divides and re-forms in the new cells. When this happens in sperms or egg cells, it causes changes that can be passed on to a child. The appearance of haemophilia is an example of a mutation in a single gene.

 

 

 

 

 

 

 

What are dominant genes?

                      Although genes from both parents are mixed together at fertilization, some genes have a more powerful effect than others. These dominant genes override the effects of others which are called recessive genes. For example, if a child has a gene for brown eyes from one parent and a gene for blue eyes from the other, the child will always have brown eyes. This is because the gene for brown eyes is a dominant gene.

 

 

 

What is Down’s syndrome?

                   Down’s syndrome is a condition caused by the appearance of an extra chromosome in the embryo. This extra chromosome is repeated in all the body cells. A child with Down’s syndrome has 47 chromosomes instead of the usual 46, and this difference causes physical and mental changes.

 

 

 

 

How do I inherit genes from my parents?

                          Sperms and egg cells contain only half of the normal number of chromosomes. During fertilization these chromosomes combine to make up the usual total of 46 chromosomes. This means that half of a baby’s genetic information comes from the mother, and half from the father, mixing together their characteristics in a random order. In this way you develop a mixture of characteristics from both parents.

 

Are there such things as ‘bad’ genes?

                          Some genes cause us to inherit a harmful condition. The disease haemophilia is an example of such a ‘bad’ gene. It is handed down through the male side of the family but the disease is carried on the female, or X, chromosome. This means that women pass the disease on to their sons, even though they are not affected themselves. Haemophilia will not affect a female unless both parents have the gene, which is a very rare occurrence.

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What are chromosomes?

                      Chromosomes are tiny threads that are present in all cells apart from red blood cells. They contain all the information for an entire person to develop. There are 46 chromosomes in each cell. They come in 22 pairs, plus another special pair that determine a person’s sex. Chromosomes are found in the cell nucleus, but they are not normally visible under the microscope except when a cell is dividing.

 

 

 

What are genes?

                   Genes are short sections of a chromosome. Each gene carries the instructions for a specific characteristic, such as eye colour, each carries the instructions for making a protein that will form a part of a living cell.

Many of these genes work with other genes, so it is difficult to say what effects they will have. Scientists are currently studying all the genes in a human cell, which will give them the complete blueprint for a human being.

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What is a caesarean section?

                            Sometimes a baby cannot be born normally through the vagina and so it has to be surgically removed from the mother’s womb. This operation is called a caesarean section, and it is normally only carried out when there is a risk to the baby or the mother. A common reason for carrying out a caesarean section is when the umbilical cord becomes wrapped around the baby’s neck during the birth. There is a risk to the baby’s life if it is not born very quickly.

 

 

 

 

 

How are twins produced?

                      Twins may be absolutely identical, or only as similar as a typical brother or sister. Identical twins are produced when the embryo splits into two in the early stages of its development. This produces two identical children of the same sex. Non-identical twins are produced when two eggs are released at the same time, and both are fertilized. They can be the same sex, or brother and sister. One in 83 pregnancies results in twins. Identical twins look so alike they can only be told apart by fingerprints.

 

 

 

 

Why does a new baby have a hole in the top of its skull?

                      Even a tiny baby’s head is too large to pass through the gap in the mother’s pelvis. A baby’s head is flexible so it can be squeezed out of shape as it passes through the pelvis. This is made easier because some bones in the baby’s head are not knitted together, and some of them are still made of rubbery cartilage. The tip of the baby’s head is the last place where the bones join. For weeks or months after the birth there is a soft patch on top of the skull where the bones have not yet fused together.

 

 

 

 

What is a baby like just before birth?

                             At the 26th week, the baby is big enough to survive if it is born prematurely. It weighs about 1.5 kg, and from now on it increases in weight ready to be born. Soon the baby turns over into a head-down position ready for the birth, which usually takes place at around the 38th week.

 

 

 

 

 

How is a baby born?

                       The mother starts to feel strong tightening pains, called contractions, in her womb when the birth is near. These contractions become stronger, and the neck of the cervix starts to open. As the contractions continue, the baby’s head moves down. The membrane around it breaks, releasing fluid out through the vagina. After hard pushing, which can last for several hours, the baby emerges through the cervix and the vagina. The baby is still attached to the umbilical cord, which is tied off to prevent bleeding. Soon after, the placenta is pushed from the mother’s womb.

What is a foetus?

After the first eight weeks of pregnancy, the developing baby is called a foetus. By now all of its major organs have formed and it is growing at a very fast rate. By the 16th week the foetus starts to move about, and by the 20th week it may already have eyebrows and fingernails. It weighs approximately 350 g.

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What happens after fertilization?

                    The fertilized egg begins to divide as it travels down the fallopian tube towards the womb. By the time it enters the womb it has divided into a ball of about 100 cells. It settles on the wall of the womb and sinks into the surface, becoming firmly fixed. At this point, the female is pregnant.

 

 

 

 

What is an embryo?

                     For the first eight weeks of a pregnancy, the developing egg is called an embryo. A liquid-filled bag develops around the embryo to protect it. By the fourth week of pregnancy the embryo is the size of a grain of rice. It has a head and a tail, and the beginning of limbs, and its tiny heart begins to beat. The placenta is the embryo’s life-support system. It is a red, flattened organ that becomes deeply embedded in the wall of the womb. The placenta extracts food substances from the mother’s blood and passes waste material from the embryo back to the mother for disposal. The placenta is connected to the developing baby by a thick umbilical cord, which contains large blood vessels.

 

 

 

What is a period?

                          Each month, an egg cell is released from a female’s ovaries. When this happens, the lining of the womb changes so that it is ready to receive the egg if it is fertilized.

                           If the egg is not fertilized, the womb lining breaks down, and blood and tissue pass out through the vagina. This process is called a period, and it happens about every 28 days. The time from one period to the next is known as the menstrual cycle. Hormones from the pituitary gland and the ovaries control the menstrual cycle, which may he irregular. However, it usually settles into a regular cycle.

 

 

 

How does fertilization take place?

                         It takes two days for an egg cell to travel along the Fallopian tube. During this time it may be fertilized. The male’s penis places sperms in the vagina, and about 300 million sperms are released. Some sperms pass through the neck of the womb and swim up into the uterus. They enter the Fallopian tube where some will meet the egg. Only one sperm may wriggle inside and fuse its own nucleus with the nucleus of the egg.

 

 

 

 

What is contraception?

                       Contraception prevents fertilization from taking place. There are several different contraceptive methods. One method involves a contraceptive device called a condom. It is a rubber covering, or sheath, that is worn over the penis to stop the escape of sperms. The contraceptive ‘pill’ is a drug taken regularly by a female. It prevents the ovaries from producing ripe eggs, or stops the eggs from developing.

 

How are sperms made?

                        Sperms, the male sex cells, develop in the testes. They are stored for several days until needed. The testes contain long tubes called the seminiferous tubules, which are tightly coiled. Sperms are produced continuously in these tubes, then passed to the epididymis and stored in a large duct called the vas deferens. Here liquid is added to the sperm to make a milky fluid called semen. It is stored in pouches called seminal vesicles. During sexual intercourse the seminal vesicles contract and force out the sperm. It passes out of the penis and into the female sexual organs.

How many sperms are there?

                     Up to 100 million sperms are produced every day by the male. If they are not released, they are soon destroyed and replaced. Sperms look like tiny tadpoles, with rounded heads and long lashing tails. They use their tails to swim at a speed of about one cm per minute. The head of the sperm contains the nucleus, which will join with the egg cell during fertilization.

 

 

What is a human egg cell?

                         The ovaries of a newborn baby girl contain about 250,000 egg cells. By the time she reaches maturity, there will be about 10,000 remaining. However, only a few of these eggs will be used — one each month throughout the years when she can have a baby. Every 28 days, a swelling appears on an ovary. This releases the egg cell in a process called ovulation. The egg cell then enters the fallopian tube. The swelling it emerged from produces hormones to prepare the womb to receive the egg.

 

 

What are the female sex organs?

                        The female sex organs are inside a woman’s lower abdomen. The womb, or uterus, is a pear-shaped muscular organ from which two ‘horns’ run out sideways, ending with the Fallopian tubes. The neck of these tubes is funnel-shaped, and the funnels are cupped around the ovaries where the egg cells are produced. The neck of the womb leads to a short tube called the vagina, leading outside the body. Urine passes out through a short tube, the urethra, near the mouth of the vagina.

 

 

 

What are the male sex organs?

                         The male sex organs are the testes and the penis. The two testes are contained in a skin sac called the scrotum, hanging beneath the groin. A tube called the urethra, which is connected to both the bladder and the testes, runs through the middle of the penis. The urethra is used to pass urine and also, at a different time, to pass sperms out of the man’s body. During intercourse the penis becomes stiff as blood is pumped into a cavity called the corpus cavernosum.

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Which gland helps with digestion?

                         The pancreas is an important gland that helps with digestion. It also controls sugar levels within the body. The pancreas produces digestive enzymes that flow into the intestine during the process of digestion. The pancreas also functions as an endocrine gland, releasing the hormone insulin into the blood. Insulin helps the body’s cells to use glucose, which is essential in the production of the energy that powers cells.

 

 

 

Which hormone affects growth?

                        Growth hormone, one of the most powerful hormones in the body, is produced in the pituitary gland. Growth hormone causes growth throughout childhood and adolescence, and it also affects the way that food substances are used to build new tissues. It stimulates cells throughout the entire body, and also causes the liver to produce special substances that activate bone and muscle growth.

                        Growth hormone is produced in differing amounts during adolescence, which is why children have a series of ‘growth spurts’ when they grow very rapidly. In adults, growth hormone acts mainly to maintain and repair the tissues.

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