Science - Biology

• Charles Darwin's theory of evolution by natural selection:

  • Organisms in a species show wide variation in characteristics
  • Organisms compete for limited resources in an ecosystem
  • Organisms with the most suitable characteristics for the environment are more likely to be successful competitors and survive
• Successful organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring
• Over time, beneficial characteristics become more common in a species as it evolves
• New discoveries have helped develop the theory of evolution:

  • Darwin's theory wasn't perfect due to the lack of scientific knowledge available at the time
  • The discovery of genetics supported Darwin's theory by explaining how organisms born with beneficial characteristics can pass them on
• Cystic fibrosis is caused by a recessive allele, affecting cell membranes and resulting in the production of sticky mucus in the air passages and pancreas
• The allele causing cystic fibrosis is carried by about 1 in 25 people, and individuals with only one copy of the allele are known as carriers
• For a child to have cystic fibrosis, both parents must either be carriers or have the disorder themselves
• Polydactyly, an inherited genetic disorder where a baby is born with extra fingers or toes, is caused by a dominant allele and can be life-threatening
• During IVF, embryos can be screened for genetic disorders by analyzing their genes before implantation, a controversial practice with ethical considerations
• Variation in organisms of the same species can be genetic, caused by differences in genotype, which includes all the genes and alleles an organism has
• Mutations are occasional changes in an organism's DNA that can be inherited, leading to genetic variants and sometimes significant effects like cystic fibrosis
• Chromosomes determine whether you're male or female:

  • You have 23 pairs of chromosomes, with 22 of them being matched pairs controlling characteristics.
  • The 23rd pair is labelled XY or XX, which determines your sex:
  • XY = male characteristics
  • XX = female characteristics
• Genes control characteristics you develop:

  • You inherit genes that control characteristics.
  • All genes exist in pairs, with two versions (alleles) of every gene in your body - one on each chromosome in a pair
• Organisms can be homozygous (two alleles for a particular trait are the same) or heterozygous (two alleles for a trait are different):

  • If two alleles are different, only one can determine the characteristic present, e.g., eye color
• For a recessive trait to show, both alleles have to be recessive:

  • A recessive trait is only displayed if both alleles are recessive
• To display a dominant allele, only one needs to be dominant as it overrides recessive alleles:

  • Your genotype is the combination of alleles you have
• Punnett squares can show possible alleles of offspring:

  • Example: BB for brown eyes, bb for blue eyes
• Asexual reproduction produces genetically identical cells with only one parent, resulting in offspring that are generally identical to that parent
• Asexual reproduction occurs through mitosis, where a new cell has exactly the same genetic information as the parent cell, creating a clone
• In asexual reproduction, there is no fusion of gametes, no mixing of chromosomes, and no genetic variation between parent and offspring; the offspring are genetically identical to the parent, known as clones
• Some plants and animals reproduce asexually
• Sexual reproduction produces genetically different cells through the combination of genetic information from two organisms, resulting in offspring that are genetically different from either parent
• In sexual reproduction, the mother and father produce gametes by meiosis, with each gamete containing 23 chromosomes, half the number of chromosomes in a normal cell
• During fertilization in sexual reproduction, the sperm and egg fuse to form a cell with the full number of chromosomes, inheriting features from both parents
• A gene codes for a specific protein and is a small section of DNA found on a chromosome
• DNA, found in the nucleus of animal and plant cells, contains genetic material and instructions to put an organism together, determining inherited characteristics
• Chromosomes are long molecules of DNA, with DNA being the chemical that all genetic material in a cell is made of
• The human genome allows scientists to identify genes related to inherited diseases and trace the ancestry of populations
• Meiosis produces cells with half the normal number of chromosomes, ensuring genetic diversity in offspring
• Adrenaline is a hormone released by the adrenal glands, located just above the kidneys, in response to stressful situations where your brain perceives fear or stress and sends nervous impulses to the adrenal glands, which respond by secreting adrenaline
• Adrenaline prepares the body for "fight or flight" by increasing the supply of oxygen and glucose to the brain and muscles, enhancing heart rate and muscle strength
• Thyroxine is a hormone produced by the thyroid gland, playing a crucial role in regulating the body's metabolic rate and influencing growth and development
• Thyroxine is regulated by a negative feedback system in the body, where the levels of hormones in the blood are controlled to maintain them within normal ranges
• Hormone release can be influenced by negative feedback systems, where the body regulates hormone levels in the blood to maintain them within normal ranges
• In the menstrual cycle, there are four stages:

  • Stage 1: Menstruation starts, the uterus lining breaks down for about four days
  • Stage 2: The uterus lining builds up again from day 6 to 14, forming a spongy layer full of blood vessels, ready for a fertilized egg
  • Stage 3: Ovulation occurs around day 14, where an egg is released from the ovary
  • Stage 4: If no fertilized egg is present by day 28, the spongy lining breaks down, and the cycle starts again
• Progesterone, produced in the ovaries, maintains the thickness of the uterine lining during the second half of the menstrual cycle
• During puberty, sex hormones trigger secondary sexual characteristics such as facial hair in men and cause eggs to mature in women
• In men, testosterone is the main reproductive hormone, produced in the testes, stimulating sperm production

  • In women, the main reproductive hormone is estrogen, produced by the ovaries, influencing physical changes and also involved in the menstrual cycle
• FSH (follicle-stimulating hormone) is produced in the pituitary gland, stimulating the ovaries to produce estrogen
• Hormones like progesterone can be used to reduce fertility by preventing the release of an egg and thickening cervical mucus to prevent sperm from reaching an egg