6.1 Reproduction (Combined) Flashcards
What is sexual reproduction?
A type of reproduction involving the production of gametes by meiosis. A gamete from each parent fuses to form a zygote. Genetic information from each gamete is mixed so the resulting zygote is unique.
What are gametes?
● Sex cells (sperm
and egg cells in animals, pollen and
egg cells in flowering plants)
● Haploid (half the
number of chromosomes)
What is meiosis?
● Form of cell
division involved in the formation of gametes (non- identical haploid cells) in reproductive organs
● Chromosome number is halved
● Involves two divisions
What must occur prior to meiosis?
Interphase - copies of genetic information are made during this process
What happens during the first stage of meiosis?
- Chromosome pairs line up along the cell equator
- The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled in random, creating variation).
- Chromosome number is halved
What happens during the second phase of meiosis?
- Chromosomes line up along the cell equator
- The chromatids are separated and move to opposite poles of the cell
- Four unique haploid gametes are produced
Why is meiosis important for sexual reproduction? (2)
● It increases genetic variation
● It ensures that the zygote formed at fertilisation is diploid
Describe fertilisation and its resulting outcome
Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells).
As the embryo develops, cells differentiate.
Describe the circumstances in which malarial parasites reproduce sexually and asexually
Sexual reproduction occurs in the mosquito whilst asexual reproduction occurs in the human host
Describe the circumstances in which fungi reproduce sexually and asexually
Asexual reproduction is by the spores. Sexual reproduction to give variation.
Describe the circumstances in which plants reproduce sexually any sexually
Sexual reproduction to produce seeds. Asexual reproduction by runners (e.g. strawberry plants or bulb division e.g. daffodils)
What is DNA?
A double-stranded polymer of nucleotides round to form a double helix. It is the genetic material of a cell which is found in its nucleus.
Define genome
The entire genetic material of an organism
Why is understanding the human genome important?
The whole human genome has been studied and is important for the development of medicine in the future
● Searching for
genes linked to
different types of
disease
● Understanding and treating inherited disorders
● Tracing human
migration patterns from the past
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of genes.
How many chromosomes do human body cells have?
46 chromosomes (23 pairs)
How many chromosomes do human gametes have?
23 chromosomes
Define gene
A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.
What are alleles?
Alleles are different versions of the same gene
What is a dominant allele?
A version of a gene where only one copy is needed for it to be expressed
What is a recessive allele?
A version of a gene where two copies are needed for it to be expressed
What is meant when an organism is homozygous?
When an organism has two copies of the same allele (two recessive or two dominant).
What is meant when an organism is heterozygous?
When an organism has two different versions of the same gene (one dominant and one recessive).
What is the phenotype?
The visible characteristic
What is a genotype?
The genes present for a trait
How are dominant alleles represented in a punnett square?
They are represented using uppercase letters
How are recessive alleles represented in a punnett square?
They use the lowercase version of the same letter as the dominant allele
PKU is a recessive condition. Two heterozygous parents (Pp) have offspring. Predict the proportion of offspring that will have PKU.
75% chance of normal phenotype
25% chance of PKU phenotype
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one
What is an inherited disorder?
A disorder caused by the inheritance of certain alleles
Give two examples of inherited disorders
● Polydactyly (having extra fingers and toes), caused by a dominant allele
● Cystic Fibrosis (a
disorder of cell
membrane), caused by a recessive allele
How are embryos screened for inherited disorders?
During IVF, one cell is removed (from an 8 cell embryo) and tested for disorder-causing alleles. If the cell doesn’t have any indicator alleles, then the originating embryo is implanted into the uterus.
What are the ethical issues concerning embryo screening?
● It could lead to
beliefs in society that being disabled
or having a disorder is less human or associated with inferiority.
The destruction of
embryos with inherited disorders is seen by some as murder as these would go on to become human beings.
● It could be viewed
as part of the concept of designer babies as it may be for the parent’s convenience or wishes rather than the child’s wellbeing.
What are the economic issues concerning embryonic screening?
Costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary.
What are the social issues concerning embryonic screening?
● Social care for children with inherited disorders may need to be considered if parents are unable to provide care.
● If an embryo is
found to have an inherited disorder and is terminated, this can prevent a child and its parents from potential suffering in the future due to the disorder.
What is gene therapy?
The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele.
What are the ethical issues concerning gene therapy?
● Some people believe it goes against God as people are ‘playing God’
● The introduced genes could enter sex cells and be transferred to future generations
What are sex chromosomes?
A pair of chromosomes that determine sex:
● Males have an X and Y chromosome
● Females have two X chromosomes
Why does the inheritance of a Y chromosome mean that an embryo develops into a male?
Testes development in an embryo is stimulated by a gene present on the Y chromosome
A couple have a child. What is the probability that the child is female?
50%
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome
Why are the majority of genes found on the X chromosome rather than the Y chromosome?
The X chromosome is bigger than the Y chromosome so more genes are carried on it
Why are men more likely to show the phenotype for a recessive sex-linked trait than women?
● Many genes are found on the X chromosome that have no counterpart on the Y chromosome
Women (XX) have two alleles for each sex-linked gene whereas men (XY) often only have one allele therefore only one recessive allele is required to produce the recessive phenotype in males.
