The correct order from oldest to youngest is:
- Dryopithecines (~15–9 million years ago) - early ape ancestors, before the human-chimp split
- Ardipithecus ramidus (~4.4 mya) - earliest known hominin on the list
- Australopithecus afarensis (~3.9–2.9 mya) - famous "Lucy" species
- Homo habilis (~2.4–1.4 mya) - first tool-using Homo
- Homo ergaster (~1.9–1.4 mya) - early African Homo erectus
- Homo heidelbergensis (~700,000–200,000 years ago) - direct ancestor of Homo sapiens
So the sequence to drag into order is:
Dryopithecines → Ardipithecus ramidus → Australopithecus afarensis → Homo habilis → Homo ergaster → Homo heidelbergensisThe question asks which sex chromosome sets are possible in children from a normal man (XY) × normal woman (XX) cross.
Working it out
A normal man produces sperm: X or Y
A normal woman produces eggs: X only (she has no sex chromosome divergence)
Possible combinations:
| Egg | Sperm | Child |
|---|---|---|
| X | X | XX (normal female) |
| X | Y | XY (normal male) |
Answer: ✅ XY and ✅ XX
The other options require chromosomal non-disjunction or extra chromosomes from one of the parents, which would constitute a "sex chromosome divergence" - excluded by the question's conditions:
- X0 - requires loss of a chromosome (Turner syndrome)
- XXX - requires non-disjunction giving an XX egg
- XXY - requires non-disjunction giving an XX egg + Y sperm, or X egg + XY sperm
- XXXYY - requires multiple non-disjunction events
Select: XY and XX## Missense Mutation
A missense mutation is a point mutation (single nucleotide substitution) that changes a codon so that it codes for a different amino acid than the original.
Evaluating each option:
- ❌ Formation of a nonsensical codon - That describes a nonsense mutation (creates a stop codon)
- ❌ Codon synonymous codon formation - That describes a silent/synonymous mutation (same amino acid)
- ✅ Changing the meaning of a codon - Yes, "missense" literally means the codon's meaning changes
- ✅ The new codon encodes a different amino acid - Yes, this is the direct consequence of missense
- ❌ Absence of any change - That would be a silent mutation
- ❌ Formation of a new karyotype - That refers to chromosomal/genomic mutations, not point mutations
Answer: ✅ "Changing the meaning of a codon" and ✅ "The new codon encodes a different amino acid"
These two are both correct and describe the same event from different angles - the codon's meaning changes because it now codes for a different amino acid. Classic example: sickle cell disease, where GAG → GUG changes glutamic acid to valine in hemoglobin.## Match the class of animals with the leading brain region
This is based on the concept of encephalization - which brain region is the dominant/most developed integration center in each vertebrate class:
| Class | Leading Brain Region | Reasoning |
|---|---|---|
| Class Reptiles | Forebrain floor (striatum) | In reptiles, the striatum (basal ganglia/corpus striatum) is the dominant brain region - they lack a developed cortex |
| Class Mammals | Cortex of the large hemispheres | Mammals are defined by a highly developed cerebral cortex, which dominates all higher functions |
| Class Amphibians | The intermediate brain | Wait - actually Amphibians' leading center is the midbrain (tectum/optic lobes dominate) |
Corrected matching:
| Class | Leading Brain Region |
|---|---|
| Class Reptiles | Forebrain floor (striatum) |
| Class Mammals | Cortex of the large hemispheres |
| Class Amphibians | Midbrain |
The logic (evolutionary progression):
- Fish - medulla/hindbrain (posterior brain)
- Amphibians - Midbrain (optic lobes are dominant)
- Reptiles - Forebrain floor / striatum (first emergence of forebrain dominance)
- Birds - striatum (highly developed)
- Mammals - Cerebral cortex (neocortex dominates)
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